Because there's value in a brand name, they are using more expensive batteries (or they were, I think just recently they started shipping LFP), and you're paying for the plug-and-play convenience.

Buying cells from CATL, adding a BMS, and putting all in a case is easy but still not trivial. Definitely not plug and play. You can absolutely get dirt cheap LFPs (like other people, I hang out on diysolarforum.com too), but it is not a competitor with the Powerwall unless you want something to tinker with or are simply too budget constrained to buy the brand name product.