HTTP/2 is quite featureful and provides most of the advanced needs that gRPC has; HTTP/2 is pretty similar to what we would make ourselves if gRPC used TCP directly.

Using HTTP/2 though also allows fitting into the wider web ecosystem of proxies, reverse proxies, load balancers/TLS terminators, and developer familiarity.

There wouldn't be much benefit to using HTTP/1.1, but HTTP/2 makes it easy to pipeline and multiplex requests over a single TCP connection.

Not having to worry about calculating the length of the message before transferring it?