field hospitals are a noob trap sadly. The amount of manpower they save at the 1945 level is roughly on par with the amount that they cost to implement on the division, on top of not improving combat stats directly at all (might even argue that they make a division worse)
for small countries, you dont have the ic to use them (or the manpower really), and for larger countries you have the manpower
the most effective way of lowering your losses is using tanks (and mechanized to an extent) the issue here is that if you have bad inf then the ic could be better spent making it less bad, because even just losing less hard on every battle will cause you to lose less, and the options that enable that also make your divs deal more damage back and tend to also let you win more battles, field hospitals dont do that
and, to point this out again, it's only at that crazy 1945 level that field hospitals break even on the manpower spent to field them in the first place, before that you are losing more than you would if you didnt have them at all
on top of all that, field hospitals only really have value in situations where either you or your opponent are doing mass battleplaned offensives, one of you has to be constantly pounding the other's line. And if that is that case, the best way to save on manpower is to just stop doing that and focus on targeted attacks on the enemy lines with specialised units (tanks, mechs, specials) using the very same ic that wouldve gone to make those field hospitals in the first place