Well, I think there's value to both points - how much of the maps code would you need to be able to get something working, and how much work would it take to implement all the custom internal dependencies that would probably be missing?

I'm sure there are very complex valuable pieces of code within the maps codebase (or any other), but it would be a fairly massive task finding those pieces, extracting them, and getting them to work properly in a different codebase...

The data is way more valuable than the codebase. Being able to display a bunch of image tiles is cool, but not a mysterious unsolved problem.

Why doesn’t Google open source it then?

Open sourcing something requires extra work.

Open up their whole code base for research purposes?

It's a huge endeavour, because the company would have to convince their stakeholders that this is not harmful for the company competitive advantage. Try to convince your company executives to opensource their code. If it's easy - then share your reasoning, maybe the world will listen, but most likely not, as secrecy is generally perceived as competitive advantage.