HTTP is pretty terrible at cancellation, retry, cluster load balancing, in-channel health checks, etc. Some form of RPC is better suited for non-web facing APIs.

And yeah, I agree that it's absolutely harder to test. (Integration test especially.) But you gain a lot for the trouble: independent deployability, scalability, dynamic traffic shifting, easy sevice boundaries and ownership (data stores, state, downstream deps, etc.). For a large company, this makes a lot of sense.

Edit: microservices are not the easy path.