It's been a month but I'll try to answer. The truth is that each of the two kingdoms that you mention (Antigonids and Seleucids) went through different circumstances that hindered them from raising massive armies one after the other.

The Roman ability to rely on the mostly-reliable Socii ally system was a deciding factor. Although the Successor Kingdoms were able to raise armies faster than most people think.

The reason the Successor Kingdoms (mostly Seleucids and Ptolemies) didn't have the capacity to mobilize as many men as Rome is the fact that the core and most important part of the army (phalanx and usually heavy cavalry) was made up of Greek (all over Greece including Macedonia) colonists, although the Seleucid Kingdom used Median cavalry too, as their horses were known for their large size.

The Antigonids had natives that they used in the armies (no need for colonists), and in their fights with Rome made use of Illyrian and Thracian contingents (horsemen mostly) and the leukaspides (white shields, probably armed like thureophoroi). These able, male natives were trained and expected to fight in the army from around the age of 18-20, but in the second- third Macedonian war boys from the age of 15 were conscripted.

These colonists (called klerurchs, from kleros meaning plot of land) were given land (more like were loaned), in exchange for military duties and service to the army. Most of these clerurchies were around Syria for the Seleucids (Apamea is usually were most of the military training was happening and that's where they kept their elephants too). An interesting thing to note is that the Late Roman Empire made use of a similar system to this (in the form of the pronoia).

When the klerurch soldiers retired from old age their sons were expected to serve in the army of the King. What this created was a bond between the King (rightful Seleucid king/ heir), and the soldiers. There are accounts of soldiers refusing to fight in the side of a usurper/ pretender when he was going up against the rightful king (eg Antiochus III against Molon).

Another problem that the Successor Kingdoms faced was that they were foreign rulers (Greeks) ruling over a vast number of nationalities. That was the reason the Kings didn't want to allow natives to join the army (only used for light infantry/ cavalry). This was done as a measure to prevent rebellions, as giving natives training and equipment was only asking for trouble in the minds of the Successors (I'll talk about it more later).

The first time natives were allowed to join the Successor armies in massive numbers was the battle of Raphia. In that battle the Ptolemies allowed around 20,000 native Egyptians to be trained and equipped as phalangites to raise their numbers.

The battle was a success, as the Ptolemaic phalanx defeated their outnumbered opponent (Antiochus' inability to be the hammer to the anvil was another reason).

Following the battle, however, the confident natives decided to revolt from Ptolemaic rule and created their own Kingdom (revolt of Upper Egypt). It took decades and thousands more died for the revolt to be fully dealt with.

Even though Antiochos lost the battle of Raphia and a good part of his army was destroyed, within a few years he was able to raise a massive army (Polybius I think said 100,000 soldiers) and undertook his famous Anabasis to reclaim the eastern satrapies and restore order. His campaign points to the fact that even after a massive defeat the Successors were still able to raise armies.

In the second Macedonian war (that led to the Cynoscephalae disaster), Philip was able to raise an army of around 25,000, the majority being phalangites, who were cut down by the Romans (when Antiochus invaded Greece, he was the one to "bury" the bodies, that were apparently left to rot on the field, their bones bleached by the sun).

Even after this defeat, and some pretty strict conditions imposed to Philip V (even though they were not always obeyed by him or the other kingdoms), his son, Perseus, was able to raise an army twice as big as his father, and apparently had the money to feed his army for decades (even after losing much of Thessaly, a very important part of the Macedonian armies, as they provided horsemen and access to good trading posts).

The truth is that the Antigonid Kingdom ultimately suffered from lack of willing and available population to draw manpower and hostile neighbors that joined Rome.

Another thing that should be noted is that the Successor Kingdoms didn't have as strict of a grip on the cities/ population they ruled as the Romans. Greek cities under the successor Kingdoms (especially the Seleucids) had lots of freedom to make their own decisions and were only expected to acknowledge the superiority of the King and maybe pay some tax (didn't have to provided men in contrast to the Roman cities of Italy where socii were expected to provided certain amount of troops).

Many cities were given "freedom" called "eleutheria kai demokratia" (freedom and democracy or autonomy) in exchange for help or even out of the kindness of the King. That's what Alexander the Great did to the Greek cities of Asia Minor, relieving them from Persian tribute (or any tribute at all).

Cities were allowed to mint their own coins that had nothing to do with the Kingdom/ King, showing us that this degree of freedom existed (which didn't happen in Italy under the Romans).

Now, let's get to my favorite Diadochi kingdom, the Seleucids.

In contrast to what most people think, the terms imposed by the Romans on the Seleucids, were not the deciding factor to the ultimate weakening of the Kingdom.

The treaty imposed a heavy monetary payment of around 15,000 talents (all together) to be paid in installments, a limit to the amount of ships, the King had to give up all of his elephants and was not allowed to raise soldiers or occupy anything west of the Taurus mountains.

Ultimately, following the death of Antiochus III only a small amount of money was paid, soldiers/ mercenaries were raised west of Taurus, a large amount of ships were built for the Seleucid navy and elephants were also used. The Romans were not strict in imposing the terms, following the death of the King, the treaty was pretty much considered empty.

A few years after the death of Antiochus, his son Seleucus IV was able to raise a sufficiently-large army to invade Pergamon and assist his ally. Antiochus IV Epiphanes (brother of Seleucus), was able to raise a large force, invade Egypt (including Cyprus) and capture all of it except Alexandria (the first Seleucid king to do so).

He also "showed off" his army in the Parade of Daphne (a large army for the time, including big contingents of mercenaries, preparing for his expedition east). He was able to continuously send smaller armies to deal with the Jewish revolt.

The years that followed Antiochus IV's death is truly what dealt the significant blow to Seleucid strength. That is, continuous, civil wars.

Demetrius I Soter, after escaping from Italy, became the Seleucid King. He found a Kingdom, that was very hard to rule, as some satraps refused to obey or acknowledge him (mainly the upper satrapies, led by Timarchos). However, he was able to raise a big enough army to defeat him, reclaim the Kingdom, while also sending a big army (around 25,000) to deal with the Jewish revolt (Battle of Elasa). He was deposed/ killed by a supposed son of Antiochus IV, after fighting 2 big battles.

Lots of intrigues and civil wars followed, a big disaster that weakened the empire was the capture of Demetrius II by the Parthians (even after winning a few battles, the Parthians isolated and captured him, unlike what most people think the Seleucid armies were very able to defeat the Parthians, and won most pitched battles against them).

Yet another civil war happened, leading to a bloody finale between the rightful King and a pretender (Diodotus Tryphon). In the end, the King, now being Antiochus VII Sidetes won, he then retook Jerusalem and the surrounding cities with his army (the Jewish people were supposedly impressed by his army and accepted his strict peace terms, having to bring down Jerusalem's walls and pay tribute).

After raising yet another big army (the Seleucids military strength was now based in Syria only, as Media and Mesopotamia were lost), in the 10's of thousands (some say up to 100,000, although that is most likely exaggerated), he retook Babylonia, Media and Mesopotamia from the Parthians, winning three battles. However, after splitting his army into winter garrisons, he was ambushed and fought to death surrounded only by his royal guard.

This was a large blow, as historians at the time tell us that his army was so big that after its defeat, every single person in Syria was grieving as they all lost at least a relative.