The simple answer is that : well, it's not that Romans got Germanized, it's that Bariarians were Romanized. While French might be considered (with Retho-Roman speeches) the most "Barbarized" Romance language the actual Germanic influence is pretty much limited to phonology and a bit of vocabulary. And it's even less important in other Romance languages.At the contrary, English is a Germanic language with an important phonologic and lexical influence from Romance (mostly trough Norman influence), but is still a Germanic language as it follows the same rough lines than Frisian, Dutch or German.

A more complete answer would require us to look at what these Franks, Goths, Burgundians, etc. were in Late Antiquity.First, we can't really consider them as tribes, in the anthropologic or political meaning of the term : a tribe is a rather small human group which can often be considered without this much of a clear hierarchy (at least in a first time, rather) tied by (real or percieved) genealogical group.Germanic peoples as they entered in direct contact with Rome by the IInd/Ist centuries BCE weren't really this anymore, or more precisely, tribal entities survived but were incorporated into larger bodies with a more apparent hierarchy which we could call chiefdoms (that said, any social-political evolutionary model is debated and historiographic rather than historic, so let's exercise caution there) a society on which a chief presides rather than controls (at least in a first time, again) and a bit more territorially defined.

(Neither should Germanic be systematically understood as an ethnic concept : up to the Ist century AD, several Germanic people were at least partly Celticized and probably Celts for a lot of them. German is firstmost, even if it ended being largely Germanized in the cultural sense, a geo-social term for the ancients).

When Romans deal with Batavi, Ubii, Cherusci, Chatti, etc. on the Rhine, they don't deal with tribes, but with chiefs or chief assemblies. Which is a phenomenon that allows Romans to more easily identify partners and possible allies, allowing payments of services, more easy trade, and subsides which flows into Germania (traces of Roman military, probably advisors, as far as Polish Silesia). This allows to reinforce the prestige and mobilisation capacities (understood as the means for a chief to divert resources either from his own household or from the community to build a fellowship and an elite) of Germanic chiefs. It shouldn't, by all means, be understood as a smooth and unilateral process (Teutoburg being an obvious example, but as well are the stress on indigenous products in Germanic tombs at some point rather than a systematical use of Roman goods), but Germanic people identified themselves relatively to the military, economical and cultural super-power they lived at their very doors.

It's interesting that, as far as the IInd century AD, Germans in Roman military service defined themselves by a double identity : a Roman military identity, the name of their unit for example,and their native identity (tribal, but not necessarily). It's is well illustrated by a stela found in Pannonia in the IVth century.

Francus ego cives, romanus miles in armis

I'm a Frankish citizen, roman soldier in arms.

By the time Franks, Goths, Burgundians, Saxons, Alamans, etc. formed as bigger ensembles from the lot of Germanic chiefdoms in the IIIrd century AD, they already were under a multi-levelled Roman influence. Moreover, these leagues or coalitions formed themselves in order to use the opportunity that presented itself when Romans had to move their armies from Rhine and Danube (they weren't "always present", so to speak) in order to raid the provinces (especially as the crisis of the IIIrd century weakened their own supply, and might have been doubled by a climatic pressure). Eventually, after several more or less successful raids (carrying slaves with them, furthering a certain Roman presence) they were generally defeated by emperors once the situation stabilized and were often deported in scattered groups (called dedicitii, even if dedicitii are more of a general term) or as civilian/military settlements (laeti) to make up for the lack of manpower. By the IIIrd century and early IVth century you then already have Germans in Roman Empire, settled by the thousands but quickly drown over by Roman culture (even if they certainly kept a lot of particularities).

In the latter period, more Barbarians settled in as either auxiliaries or refugees before the Hunnic pressure : some of these were repelled or treated as before, some other were given a renewed status, being considered as federated peoples : Franks were settled in Roman Germania; Burgundians in Germania and Raetia, Tervingi in Moesia, etc. The difference being that these treaties were made between the emperor and peoples; making them effectively border armies. Sometimes it worked well, Franks remained mostly loyal and several of their chiefs obtained high distinctions as generalissimo or consuls. Sometimes, it didn't : Goths rebelled against abusive treatment and their victory at Adrianople got them enough assertiveness that they didn't heisted to raise arms against the Empire while renegotiating their status.But they all functioned along Roman imperial lines.

By the early Vth century, the situation had deteriorated enough that Barbarians formed autonomous or independent states, made along Roman lines, while building relations with local Roman provincial elites that, at the end of the day, were more loyal to the idea of the Roman Empire than the reality of the Roman state, especially as imperial authority became a dead word beyond the Alps. In the provinces where Roman administration and structures survived best, in Spain, Southern Gaul, Africa, Barbarians basically build small Roman states.Where imperial authority collapsed but roman military and civilian (systematically mixed) survived, as in Northern Gaul it was a bit murky and when Franks took over Roman warlords and cities, they established a light-weighted Roman state (before conquering southern Gaul and basically borrowing the administrations). Where both authority and structures collapsed, as in Britain or most of Illyricum, on the other hand...It was generally settled by peoples that didn't had much contact with the Late Empire to begin with (such as Saxons*) and while you still had an important Roman substrate in their societal build-up, they were still largely Barbarian in nature.

For Franks, Goths, etc. We can see that, not only living close to the Empire even before their appearance, by entering in Romania since the IVth and early Vth century, by carrying with them a lot of provincial populations (as slaves or deserters or migrants coming in to become Barbarians), by Christianizing, by being demographically the equivalent of a drop in the ocean...Well, let's say that by the late Vth, you really have to look carefully to spot something obviously Barbarian : essentially in laws (with concepts around the Wergeld and the importance of the oath being the main examples) and language. It's more or less a given that by the late Vth century, Barbarians spoke Latin : Euric pretended not to but it was an outrageous and obvious lie for all to see, Gothic scribes might have make mistakes on purpose to pretend not being good at speaking Latin, Frankish language (whom we are not even sure of its actual existence : it might have been rather a continuum of speeches) is barely attested except in ceremonial formulas, etc.

Barbarians often had to resort to more or less made-up cultural features to distinguish themselves from the overall population : by making Barbarian laws which are essentially Theodosian Law with variations on the Roman law about Barbarians (and often written with the help of Roman scholars); by pushing front identitarian clothes or weapons (fransiscae are absent from Frankish weaponry before the VIth and probably borrowed from people remaining in Germania; and the garish Barbarian clothes are essentially a late ancient variation on bling) and inventing themselves origin stories strongly influed by Tacitus, Virgil and the Bible.

Somehow...It worked, and we came to see (in the case of Franks) long hairs and long beards as Germanic (probably taken as token of a Davidic kingship) or the tradition of sub-regni as well (while it's unknown in other Germanic peoples or even before Clovis. If only there was an example of a state being split in 2 or 3 parts for political and military purposes in Late Antiquity...)

Basically, France, Spain or Italy aren't considered as Germanic because the people that build states in Roman provinces could barely be considered as "Germanic" at this point, at least not entirely, giving their own romanization. Not that French, Spanish or Italian peoples are the direct descendent, either politically or culturally, of Franks, Goths or Lombards. The actual Barbarian element in Romance culture today is really limited, and by all means, if we have to search for national predecessors form Late Antiquity or Early Middle Ages (which is as anachronistic it could be, as Patrick J. Geary remarkably demonstrated in his Myth of Nations), then they directly come from the Late Roman culture.

^(*Anglo-Saxon is a much later term, and there's no real sign that people considered themselves as Anglians in Britain before the mid-to-late VIth century. Even Saxon is a bit pushing it, but giving that a good part of the settlement come from regions traditionally considered as part of the Saxon coalition, it's a convenient and accurate enough term.)

Good introductory reads there would consist of the aforementioned "Myth of Nations" by Patrick J. Geary; but as well Barbarian Migrations and the Roman West, 376-568 by Guy Halsall or The Fall of the Roman Empire, by Peter Heater