See my comment above. LSP servers are complicated beasts coming with a lot of specific dependencies. It is simply impossible to ship all available language servers together with Emacs. Even VSCode does not do it. For popular languages it downloads them automatically for you. But this is exactly what "lsp-mode" (the alternative to "eglot.el") does, too [1]. If you are an expert in a specific language it also should not be too difficult for you to install and setup one yourself.
My personal prediction, however, is this: Thanks to the advent of the static parser framework "treesitter" we will see language servers becoming obsolete in the long run. Treesitter enables an editor to load a parser backend as a plugin and on top of that provides a powerful query API to semantically analyse your code. This should be sufficient to implement all of what an external language server does already but inside the editor's process.
Yes it can be set up, but my point was precisely that it(LSP servers for popular languages) is not baked into Emacs, instead making them complicated external dependencies necessary to be set up to get a basic IDE functionality of code completion working.
Code completion works out of the box for example in IntelliJ for example without needing to download and install further packages.
The tree sitter approach does sound something along in that direction, that should bode well if it works out.
