>>96515223
The thing is, the Imperial Guard isn't actually that numerous. The PDF picks from amongst the top 1% of the planetary population typically in terms of physical condition, and then the Imperial Guard picks from the top 1% of those. So by the time you actually get to the Imperial Guard, you're actually dealing with the top .01% of the Imperium's manpower.

But this isn't nearly enough to actually take care of the Imperium's manpower needs, which is why even the Imperium is dealing with manpower shortages. The Imperial Guard might be the worst of the basic grunts outside of Chaos Cultists and maybe Orks, but they're still drawn from a rather restrictive pool.