The closest thing to an explanation I could find online just said “legal issues”, but didn’t go into details.

I don't think that makes sense, or at least it doesn't properly qualify the problem. BIOS is a set of baked-in software routines that mediate certain operations between software and hardware. In theory it could be reverse-enginereed and thus emulated just like the rest of the hardware is. In fact, many of the more simple systems (like 8 or 16-bit consoles) have their BIOS emulated. But for more advanced or poorer documented systems, there are, in my view, two problems with that:

• If your reversed engineered version of the BIOS has bugs (and during early stages of development, it would have a lot), the ways in which these bugs could present themselves makes the situation ambiguous, because it may be hard to know, from the symptoms, whether the bug is on the BIOS or on the hardware emulation. So developers just use the official BIOS because then if you see bugs, you know for sure the problem is on the hardware emulation. And also, reverse engineering the BIOS would require a lot of effort that developers would probably rate as low priority given they could use a perfectly functional BIOS and avoid a whole lot of other technical problems as per above. I mean, for many systems, hardware emulation is a problem already complex enough;
• Depending on the system, the BIOS code could be so simple that a reverse engineered version of it could conceivably be so close to the actual official code that it could, yes, trigger a copyright suit from the creator.