Firstly Rome depended on independent and moderately well off farmers. People on the farms suffered from chronic under-employment, the consequence when you have periods of peak effort (e.g. the harvest) followed by periods of much less work. So there's an existing pool of labour on these farms that can be spared, that can be lost without destroying Rome's ability to sustain itself (and its armies).

Secondly Rome was very unusual in that they incorporated defeated peoples into their armed forces. Subjugated states were forced to become allies of Rome, required to supply soldiers to Rome's wars, give up any foreign policy, but otherwise left to run themselves. In the Punic War era, half of each 'Roman' army was composed of allied troops from its conquered states. So Rome's casualties were being spread among the large number of allies.