Presumably there are some features of the app that won’t work that way. Surely you’re not saying you prefer a Web app just for the sake of making calls.

No, I'm just taking your argument to its logical conclusion. If you manufacture enough criteria, you can steer any discussion so that your choice is the only possible choice left. That's not how things work in reality, there are many competing factors that go into technological choices.

Minimizing round trips is an optimization with improved UX and operational cost and no real downside except marginal implementation difficulty (which itself seems like an argument for being able to share validation between the backend and frontend).

because many people wouldn't use the product or you would have to maintain multiple codebases for the various operating systems and devices (including mobile).