Because it's an enormous waste of time writing code, doing QA and managing updates for multiple different platforms. Web apps inside a headless Chromium container is how many many "mobile apps" are built these days.
It's also an enormous waste of time building really great products when you can cut corners and sell things very cheaply, maybe even create things like single-use disposable t-shirts. Why is this any different? I guess you can argue that web apps are superior or on-par with something like maybe an iOS specific app, but I'm not sure that's true.
Many of the apps I've come across perform best when they're built for the platform they operate on. I can usually (not always) tell when I'm using a web app because the experience kind of sucks.
But even if they're on par, let's say you can run your app everywhere including a smart watch - is that really the goal?
