I think this is because it’s more expensive and takes longer to build and maintain a desktop website, mobile website, and app.
If you design everything to work on mobile, you can reuse it as the desktop site. You can most likely reuse assets in the app as well.
Also, people are using their phones a lot more frequently than desktop/laptop, so mobile experience gets prioritized.
My husband’s ex-wife cheated on him with his brother and best friend. It would be completely terrible to cheat with those people even without that history, but it makes it 10x worse.