looks like rendering adblockers extensions obsolete with manifest-v3 was not enough so now they try to implement DRM into the browser giving the ability to any website to refuse traffic to you if you don’t run a complaint browser ( cough…firefox )
here is an article in hacker news since i’m sure they can explain this to you better than i.
and also some github docs
Serious answer: because we wanted interactive sites (ajax it used to be called, iirc), like most online shopping.
Less serious answer: indeed, what do we even need javascript for when we have
marqueeandblinktags?