Most (all?) LSP implementations are open source. If the same effort they put into reimplementing the compilers went into making better LSP-based solutions, LSP would run circles around custom plugins, because LSP has accurate data from the official compiler. However, I guess I know why they are not doing that: because that would benefit the competition as well.
And yeah, obviously Java story is definitely polished, but it has been under development for decades, and also Java is quite a limited language.
Actually, Microsoft's Python LSP for VS Code is proprietary[1] (as are some of their other extensions) whereas PyCharm Community Edition is open source[2].
What about other things external to the compiler like immediately recognising and mapping project configs and structures (for example, Spring, or Symfony, or...)?
Or things like "version X of the language introduces new things and we can automatically refactor your code to reflect the new ways of dealing with things"?
And yeah, obviously Java story is definitely polished, but it has been under development for decades, and also Java is quite a limited language.