Because you have to figure out what should be in it, and coordinate the distribution. It's not like there's a reference implementation of JavaScript maintained by a well-known team that you consciously install everywhere that you need it.

Node is pretty much everywhere regarding JavaScript cli and web apps (server side). As for the web it’s hard to argue for a slim library when most sites are dumping huge analytics bundle on us.

At this point, it’s just status-quo and lazyness