You build a system, the web app is supposed to be a demo of what your system can do. But since the days of Facebook, YouTube, reddit, the web app is the product itself. This is why these companies always break third party apps that connect to their API.
So you're in one of a few situations. You have to build a web app because precedent, and you don't want cost overruns. Also use case comes second to business case. And we end up where we are. It's a mess for the user, and I don't think it's sustainable, which means this problem won't exist long term.
Also your last paragraph doesn't follow to me because the whole reason these apps exist is because they are web apps. They wouldn't exist otherwise. That's the whole reason they are sustainable now in the first place. They're the default because it's the cheapest and easiest way to deliver a product for billions of users.