If you think of a commit as a Merkle tree, then a file's content is a leaf node, and thus has nothing to verify. It either exists or it doesn't. Non-existence creates usability problems, but not verifiability problems.

Of course, remote files can be used to sneak things in, but those things still have to get approved the same as any other commit content. You should not approve PRs etc. that reference remote files you haven't verified. And, while remote storage could be vulnerable to collision attacks in a way that git itself mostly isn't, git-lfs for example already uses SHA-256.