I'm philosophically aligned with you. Basically, I think this is because of where resources are invested - there's a tremendous amount of work on programming languages to make them ergonomic and enable composition of pieces. Much of that work is at its essence processing friendly syntax into some form of typed lambda calculus. Since dependently typed lambda calculus is a very handy way to encode proofs, it's easier to just adapt that all that mechanism than to develop infrastructure just for proofs.
Interop is hard, because all the axiom systems tend to be subtly different in ways that are hard for normal people to appreciate, though there is some work on it, including MM0.