Why do people think context window size is a big limitation for AIs to be able to fully replace humans as coders?

I'd say most of us keep a very compressed version of the codebase in our head and rely on code search and tools to get the details we need. Coding agents are already starting to do this using grep and other tools. They just need that initial context to get them going and then they can do the rest. Creating that initial compressed representation of the codebase they can reference to get them started would do the trick without keeping the entire codebase in the context.