It's not quite accurate to say that the Roman Empire relied on Germans to fill the lower ranks of its army, at least before 212. Typically the Romans would form battalion- or regiment-sized units wholesale from a particular territory, from troopers to high officer ranks (sometimes under a Roman commander). Roman citizens (originally only from Rome proper, but from all of Italia after the Marian reforms) formed the famous legions; the provinces formed auxiliary forces of various kinds, and the latter became more important in comparison to the former as the empire grew. They also tended to grow closer to each other in terms of organization; the first auxiliary forces used their native weapons and tactics, but that distinction eroded over time.

Also, the legions were a major public employment project in the late Republican period and the early empire, with 125,000 legionaries under Tiberius at a time when Rome was a city of a million. Manpower shortages grew severe enough by Diocletian's time that he had to institute conscription, but that may have been a consequence of more need for troops rather than declining enlistment.

If lots of people are starving but the army is well fed you would expect plenty of people to want to join the army.