Web browsers are essentially an inner platform now. Instead of taking the time to develop native apps, or even to write using cross-platform frameworks, developers prefer to trade performance/security/privacy for developer time. It's really just an acceleration of a trend that has been going on for decades (Wirth's Law) but Web 2.0 is hip and cool. You're not writing some lumbering dog of a Java application (write once run anywhere!), you're agile and using React and web sockets, you're one of the cool kids (writing a lumbering dog of a Javascript application).

https://en.wikipedia.org/wiki/Inner-platform_effect

All in all, building out applications using cross-platform frameworks seems to be a far better solution for end users, and not significantly worse in terms of developer velocity, but I think there are other reasons they are not adopted, primarily that the web makes it far easier to invade privacy and deploy dark patterns, while that behavior typically is checked by the app store approval process for "native" or cross-platform applications.