Part of the problem is that the websites need to "share" budget with the teams that build 2 separate native apps (or maybe they use something like React Native, but that still costs money that could've been used for a better mobile website).

Also, if a company has a mobile app, a bad mobile website is less of a problem. Take Takeaway.com. Their mobile site is terrible. But they have a pretty good mobile app, so why would they need to pour money into making the site any better?

But yes, (mobile) web also just needs to do better.