Once upon a time, manpower figures were more decent and you were limited in how much you could merc because these units would not update when you switched units and the pool replenished slowly.
Later, Pdox nerfed base manpower gain then bizarrely introduced the infinite manpower pool. Instantly gone were the days where it was possible to exhaust a nation of recruitable resources entirely pre-bankruptcy. I still don't see why this change was either necessary or good for gameplay.
Now you can either run 0 professionalism and use mercs with good income or run 80+ professionalism and use slacken --> hire generals to keep manpower decent. The professionalism will give you enough siege ability and if you have -attrition stuff you can probably make enough MIL to cover this while still teching late game, especially if not going revolutionary and stuffing that MIL into RT for more re-elections.
Anyway, you see more mercs from the AI since it builds less garbage buildings. With more viable choices and less dock spam, it makes more money. This is a good thing regardless of surrounding mechanics otherwise; less AI throwing is an improvement in the general sense.
Global merc pools might be more historical, but they're a bad idea. It'd be pretty easy to corner the market in game terms, effectively making it so that only the leading nations have mercs.