Increased costs and useless reinventing of apps three times, what a great future.

Both platforms have different UI guidelines. It's more on respecting the users of each platforms.

Who’s stopping you from respecting different platforms with your web ui?

I'd say it's the inherent foolishness of spending time and $$$ trying to get your web UI into the uncanny valley of looking "as close as possible to the native UI" instead of spending that time and $$$ actually using the native UI and components, but that's just me.