Because of abuse, by people who upload entire movies/pirated software as PyPI packages so they can be redistributed for free (including splitting them up into individual files so that each file can be below the limit).
The only other way around this that has any success at all is to do enough identity verification that if you’re found to be abusing it, it can be provided to local law enforcement (though usually the “verification” is a stolen credit card, and nothing is really gained). And identity verification is likely to be worse (as in, less popular) than quotas and manual increases.