The TypeScript language service is indeed extensible, but keep in mind that extensibility points can only get you so far, and JetBrains has spent the last 20 years putting a massive amount of work into a JVM-based code intelligence system. They can't just port all that to Node.js (and Go and every other language/runtime where language servers run).

Of course it would be great if the official TS language service included every imaginable IDE feature, but realistically it will always lag behind a team focused on exploring new ideas. JetBrains is going to want to experiment with new language-aware features, iterate on some of them, and throw out the ones that they aren't happy with. An open source tool shared by all editors isn't the right environment for that sort of experimentation. (They could fork the language service, though, which has its downsides but maybe is most reasonable here.)

the language server protocol doesnt support everything jetbrains does.

they can extend the server as much as they want, if the protocol doesn't support the additions, they'll be useless features unless they'll implement protocol extension, which could potentially be incompatible with the original... and they're back to the starting proposition.