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.
I’m on board with this except instead of reusable glass bottles that need to be transported around, you’re responsible for your own reusable bottle/mug/thermos and you can only get beverages from a soda fountain.