I'll reply on the Czech case.

Short answer:

Because Bohemian and Moravian rulers were ambitious enough to join the Western Cool Guys' Club and to stay relatively independent while doing their diplomacy.

Long answer:

First of all, the mountainous region around Czechia makes a natural border that is suitable for defense. Czechia is also rich in forests - another important defense factor. Even today, 1/3 of Czechia is covered in forests so imagine the share back then.

In the 1st half of the 6th century during the Migration Period, when first Slavs arrived in the present-day Czechia they met little opposition - Germanic tribes already left and those few who remained were gradually peacefully slavicized over next decades. So it was in fact the other way around in contrast to what you think - Slavs in fact settled "around" German lands. Or claimed abandoned German lands "around" actively settled German lands, if you will.

Early Bavarian nation is also formed sometime in the 1st half of the 6th century. Bohemian Slavs and Bavarians did of course experience some border skirmishes over time but a sophisticated conquest agenda wasn't likely the case from either side for the time being.

Let's have a look at serious powers in the region since the arrival of Slavs. Frankish Realm? No, it is still 2 centuries early before they were able to submit neighbouring lands to their rule; Saxony, Bavaria and Carinthia. Before that, the only known conflict between Czech/Moravian Slavs and the Frankish Realm (if we believe the only source, the so called Fredegar's Chronicle) dates to year 631/632 when Samo of Samo's Empire supposedly defeated the Frankish king Dagobert I. and secured his sovereignty.

Avar Realm? To a degree, yes. They primarily settled the Pannonian Basin with their power extending to neighbouring areas where they subdued local Slavs. Samo was supposedly able to defend his sovereignty from Avars too and even after the dissolution of his empire Bohemia and to a degree Moravia was left alone by Avars.

Fastforward to the end of the 8th century; the Frankish Realm heavily expands to the east, subduing Saxony, Bavaria, Carinthia and also the Avar Realm. What a major shift in power distribution. In these times of a temporary opportunity, the Great Moravia is born in 833. Its rulers had to juggle one more important element of European politics unlike Samo - the spread of Christianity and making a choice between following Orthodox or Latin branch. The Eastern Frankish Realm sought to subdue the Great Moravia by whatever means, either on a battlefield or through Latin Christianity. Moravian rulers managed to stay somewhat independent, yet they were subject to Eastern Frankish meddling in who would sit on the Moravian throne or even wars resulting in Moravia paying a tribute for peace.

The Great Moravia vanishes in 906, partially because of Hungarian raids but also due to immense costs of maintaining the court, satisfying nobility, defense force, maintaining fortificiations. Such expenses were costly even for the Franks with their unparalelled industry, economy and infrastructure.

With the decline of the Great Moravia, the Bohemian region grows in importance and separates itself from the Great Moravia in 895. Not only do we see the birth of a process of Bohemian (and later also Moravian) unification, we also see an important shift in politics; Bohemian rulers started to focus mainly on Bavaria of Eastern Frankish Realm. It is only logical that they also chose Latin version of Christianity as the official religion, despite old Slavic beliefs still widely spread for years to come and some not-so-very-Christian practices like slave trade still going on in future decades. It was a matter of political allegiance in the first place. "If we were both Latin Christian, they would not want to conquer us.". Not that it was super-effective, battles happened here and then and sometimes Bohemian rulers were bound to pay a tribute to cement peace.

Bohemian rulers sought to have their own bishopric to stay as independent as possible. A remarkable ruler in this sense was Boleslaus II. the Pious. He founded many churches, was allowed to found Prague's own bishopric sometime between 973-976 and also joined Bavaria in a campaign to Polabian Slav pagan territories.

By the end of the 10th century we can say that the Duchy of Bohemia was already a well recognized state, even if unstable at times and politically dependent on Bavarian and Roman rulers. This was further set in stone in the following crisis during the brief rule of Vladivoj, a weak alcoholic ruler installed by Poland. He turned to the Roman king Henry II. in 1002 with a plea to receive Bohemia "as a fief", thus formally asking if the Duchy of Bohemia could join the Holy Roman Empire. His wish was granted. One year later, Vladivoj died, probably drank himself to death. His successors couldn't bring stability to Bohemia, not to mention to break from the precedent Vladivoj created.

In any case, Bohemian rulers from then on sought goals within the structure of the Holy Roman Empire. They sought to elevate the duchy to kingdom. They sought to expand their realm. The most successful ruler in this sense was Charles IV., the king of Bohemia (1346-1378) and also the Holy Roman Emperor (1355-1378).