Because it's the best fit. They're both unique complex systems which leads to different use cases and pros/cons.
Mobile is best taken advantage of via native. It has a rock solid distribution system, which sort of doubles as a marketing platform? It's more portable. There's lots of system features exposed to native apps. Your app can be on always, collecting telemetry and pushing notifications. The net connection is not always great.
Desktop is best taken advantage of via the web. Portability is a pain for native desktop, and there's no unified distribution system. You don't need background apps to be on all the time, people spend more time with their phones anyway. There isn't as much unique hw/sw functionality. But you have the fastest net access.
Mobile is best taken advantage of via native. It has a rock solid distribution system, which sort of doubles as a marketing platform? It's more portable. There's lots of system features exposed to native apps. Your app can be on always, collecting telemetry and pushing notifications. The net connection is not always great.
Desktop is best taken advantage of via the web. Portability is a pain for native desktop, and there's no unified distribution system. You don't need background apps to be on all the time, people spend more time with their phones anyway. There isn't as much unique hw/sw functionality. But you have the fastest net access.