The same is true if you host yourself. Storage is neither free nor infinite. Sure you could scale your storage, but that costs, too. Unless you're absolutely strapped for cash, I think this just encourages good hygiene.
Frankly, there are hard problems in internal IT, but hosting a bunch of git repositories is a non-issue.
Yet all of this comes to less than a gigabyte, which is well under the limits of the free tiers at most major storage providers.
In terms of storage costs, Github for most projects is several orders of magnitude more expensive than other storage providers even if you ignore the free tiers.
I've done precisely that for personal projects in the past, as well as for a couple small teams as an intermediate step towards hosted git. I support GH mainly because they are supporting open-source... it's indirect, but I support the model.