1) nvim's built in lsp client is entirely in lua and afaik none of it is related to the nvim core. It is not inconceivable to just move it to a plugin but pretty much everyone will have to depend on it. I think for something as powerful as lsp, it might as well be bundled.
2) the built in lsp client in lua is EXTREMELY customizable and lightweight. In fact, a lot of useful features are enhanced or extended with plugins. Putting the onus on plugin writers to rewrite essentially the same functionality is not ideal.
3) The people that built the lsp client are extremely passionate about it and have contributed to neovim. tjdevries has some youtube clips and twitch stream clips where he talks about why this is a good idea. One thing that was mentioned during the stream today was that luajit and using libuv / LUV makes you feel like why not write an LSP client in nvim lua.
I personally think since it is so lightweight and since it is all in lua, it's awesome to have. And the next big thing can also be added to nvim base in lua without too much "cruft".