Well because there are still more web developers out there than mobile (native) developers. So, in any case, it would help getting more apps in their store -> more happy users -> more money
They don't want more apps, they want more exclusive apps. The more exclusive apps the harder the lockin to the platform. Apple and Google both remember mid nineties Microsoft. Lock users in and ride that to dominance. It's seems like something I should be furious about but it's hard to angry when it helps to line my own pockets with cash. I can and have written native apps for both iOS and Android so that makes me very much in demand. I'd prefer an open web to win out but IMO native apps still give a much better user experience.
Yes, but web technologies generally forbid you from doing anything remotely innovative because of the way they are designed.
Most "web apps" work worse than most apps we had two decades earlier, both in terms of design and performance. If you want people to pay for software on the app store, it helps if you put some effort into making your app responsive and exploiting the machine you are on. The only way to do this is with native applications.