But that is what they are mostly used for.

On phones, most of the compute is used to render media files and games, and make pretty animated UIs.

The text content of a weather app is trivial compared to the UI.

Same with many web pages.

Desktop apps use local compute, but that's more a limitation of latency and network bandwidth than any fundamental need to keep things local.

Security and privacy also matter to some people. But not to most.