We are fully capable of designing them. Good luck getting consumers to use them. Particularly using decentralized at every step of the process, in a way that people use Google or Facebook as a one-stop-shop for getting everything done.

If a decentralized solution is not easy to pick up, and does not offer clear everyday benefits to users over existing centralized solutions, then it will not win with users. (Just "better privacy" is not really a good everyday benefit for the average person; compare DDG usage to Google.)