Because you can ship in a few days instead of wasting weeks trying to bike shed through something as basic as css.

Use tailwind as a tool, have a component library that can be put together in a couple days max, and now you have ubiquitous styling language and don’t need to waste hours on css of all things.

There are bigger fish to fry.

Well you don't override everything but just what you need. Then devs can still use Tailwind docs on how to use the classes and all