Umm Compatibility reasons? It's probably also much easier to create certain things using a full range of tools rather than trying to implement it purely in css
Also because commonality of user experience has great value in satisfying the principle of least astonishment. It's true that a lot of web devs and designers don't get this right, but that's no reason to suggest they shouldn't be trying.
I think this work is valuable, but this is more R&D type work than a lot of people who are actually producing work with immediate value results, and tied to a business need