Exactly this, the complexity of a production compiler project is its sheer vastness. Hundreds to thousands of language features need to work together in a performant and correct manner.

Plus, the language itself is probably less than 15% of the work. You'll need a build system, a standard library, a code formatter, a syntax highlighter, support for incremental and cached builds, helpful error messages, bindings for a systems language like C, among other things.