Because LSP is not just "give me some semi-decent completions based on parsing tokens in this file". Proper LSP will treat code semantically, in case of e.g. Ru

Because LSP is not just "give me some semi-decent completions based on parsing tokens in this file". Proper LSP will treat code semantically, in case of e.g. Rust it may complete a method name from a trait method because you have the trait somewhere in scope, which comes from a third-party package which the LSP also parses, where it needs to expand some procedural macros to get to source code, etc etc. We've been spoiled by good LSPs, perhaps. But again, it's not just "parse this file please".