That is due to many factors. For one, Aetius left North Africa out to dry.
But it was supposed to have 10,000 comitatenses.
Second, North Africa was, before that point not under any serious threat, or at least, apparent threat like say, the ever troublesome Gaul and so on the list of priorities for military assets, it was fairly close to the bottom.
North Africa was under very serious threat since the very aftermath of the sack of Rome. Bad weather thwarted gothic plans but IIRC as early as 417 the Theodosian wall was built around Carhago, in anticipation of possible attack.
Third, one of the reasons the West was so chronically short of cash and manpower was to a certain extent a demographic problem, as Latifundia run by major landowners were not willing to let their tenants join the army rather than earn them money
That was a problem since the fourth century but I don't think it was the crux of the problem. The landowners had to furnish men, but many of them would cut off their thumbs rather than serve, or desert. Most citizens just didn't want to fight in the army anymore.
but those same Latifundia were increasingly reluctant to pay any taxes at all, and what taxes they did pay were often being appropriated by corrupt officials before they ever reached the treasury. Add to that the resulting ridiculous tax burden placed on the middle class and suddenly they are cutting the feet out from underneath the economy. It wasn't that the funds did not exist at all, it was that all the money in the empire was rapidly being collected and then sat upon by an increasingly small group of people.
If the problem were that great, the State could've used mercenaries to crush the senators as well as burgundians and others.
Once North Africa was lost, whatever remained of the regular Roman army ceased to exist. They were forced to turn now entirely to feodorati and other mercenaries that already had equipment and training and just had to be paid.
Essentially the regular Roman army ceased to exist in 408, but a residual force persisted.
They still had some bite, as Flavius Aetius proved when he beat the Mongols,
Mongols??!
Huns.
However the age of the Professional Roman Army in the West was over, as Africa, which was second only to Egypt in wealth as a province, was no longer available to provide food and taxes to support the army.
In fact the regular army was already gone for 20 years before the loss of North Africa. Constantius had to rely on visigoths to fight Vandals in Spain. The point is, the WRE still had adequate funds for a decent regular army even after it evaporated; it just had to use the resources to pay federates or mercenaries, mostly.
.... Maybe go back to the old mainstay of rewarding veteran soldiers with land to help secure their loyalty and help rebuild the Empire's manpower pool.
I dunooo..given the evidence that citizens would no longer serve.