Because paying 280-300mp for Volks in their current state would be fair. And mun upgrades with less than standard mun income to make these overpriced dudes even remotely useful beyond 5 mins. Much balance many wows m8.
Or you can look at it this way:
You still have basic infantry that scales well, but isn't independent of other squads(volks theme fulfilled early game).
You have a cost which prevents spam and encourages you to actually use supporting units in all stages of the game, I gave random number for the cost, but the main idea is to increase it enough to make volkspam only builds still an option, but not a no brainer choice introducing basically something they lack-a disadvantage.
You have a unit that scales thanks to upgrades(both AT and AI, so your point about them being "remotely useful" past 5 mins is completely moot), making it live up to the cost and be more independent in mid/late game. God forbid you have to actually manage your muni instead of pumping it all mindlessly into shrecks! If USF is able to deck their squads with 120mun per squad while using other upgrades and abilities, so is OKW capable of equipping multiple 90mun upgrades, which already is proven to work with volks+fusiliers builds and there is no problem with it.
So... get a bigger picture.