Sometimes we look at Apple code too, within reason and where strictly necessary.
The problem is that there are safe ways of doing this and terrible ways of doing this. When I look at vendor code, my goal is to understand what I need to do to the hardware to get it to behave how I want. I don't care how Apple's code works; I only want to use it to help answer questions about the hardware, and then build my own driver with my own approach.
However, I've seen other people (including a Corellium co-founder, in a past project that predates the company) "reverse engineer" code by literally translating it back to compilable C code, with the same logic, functions, and everything. And that is where you run into copyright problems.
And the issue is that unless you re-do all the work from scratch, it's not easy to tell from the output whether this has happened or not. I've been bitten by this in the past. And so, as the Asahi Linux project founder, I can't afford to accept contributions from any random person that are not clean-room, without having some level of trust. I have a few long-time friends in the project who I know won't screw this up, so I'm not worried about them taking a peek at Apple's drivers in Ghidra. But if it's someone I don't know, they need to build trust and we need to have a conversation about this first.
Corellium have so far refused to have any conversations with us. They never replied to any emails they were CCed on, nor have they joined our project IRC channels.
So, without being able to build trust that their approach is kosher, I can't just take their code and use it.
Practically speaking, Corellium certainly have internal documentation on the hardware that they could share (much like we're dumping info on our wiki) that would be immensely useful to us, but I doubt they'd ever share that, as it would potentially help someone compete with their product. Asahi Linux is, in this way, fundamentally at odds with Corellium: we're trying to reverse engineer the M1 and share that knowledge, while they did so to build a proprietary product and keeping that reverse engineered knowledge a trade secret is important to them.