Because JS is bad, and JS have a MASSIVE user base, so whatever they do is the web.
And because JS is on the frontend, solutions are front end, even the ones that eventually run on the (js) back-end.
Is like how people use a RDBMS but never do foreign keys, views, etc and re-invent all, poorly.
I had similar thoughts, but how does one use an RDBMS without making use of FKs? Do they put all in one huuuge table, that has all the columns and is super sparse? Or some other fever dream of bad design?