Seems like part of it would be to ensure that you will always be training new pilots.
If you have a lot of 'comfortable' pilots, you have less need to do the training, so when you fight a war and start losing pilots, you have less bandwidth to create new ones.
So there's a couple of things:
You could always recall and retrain the ones you've released if you're low on pilots, that's pretty straightforward for a wartime government if they're desperate.
It is much harder to scale the recruiting / training pipeline if it is insufficient to comfortably replace the losses you're taking. So you run your pipeline at a higher rate than necessary so that in wartime you can maintain your forces.
I think this also explains why the US would allow Boeing to sell things like advanced air-force fighters to other countries. At the surface, it makes no sense to give away your best stuff to another country. But if you think about it, it lets you run your pipeline at a higher rate, and the other guy can't replace his stuff when it starts getting blown up, you get priority. So you get to run at closer to a wartime production rate, with maintenance subsidized by other countries.
It may be cheaper to do the pilot training another way, but the last thing you want is to end up with a shortage of pilots when you actually end up needing them. It is not about cost so much as it is about winning wars and the supply chain therin.
I suspect a lot of countries don't have this policy because they have a grand total of 22 planes and no way to replace them, so if they get blown up there's nothing for new pilots to fly.
tldr; think about them resources that get expended and that you will inevitably have to create more of, rather than as highly skilled professionals