Because you may already have robust and sensible gRPC infrastructure setup and working, and setting up the correct HTTP infrastructure to take advantage of all the benefits that plain old HTTP provides may not be worth it.

If moving big files around is a major part of the system you’re building, then it’s worth the effort. But if you’re only occasionally moving big files around, then reusing your existing gRPC infrastructure is likely preferable. Keeps your systems nice and uniform, which make it easier to understand later once you’ve forgotten what you originally implemented.