Because it's not the right approach. There is no reason why building firmware cannot be done with cross compilers, so moving those directories to Meson (not because Meson can't handle them; more specifically, the issue is tying the firmware build to the QEMU build system) is going in the wrong direction.
The "Canadian cross" scenario, where you build on Linux a mingw GCC but the compiler is s390, is not even enough to describe the complexity in the case of QEMU, because there are multiple firmware for different machines.
However we already have all the infrastructure to do such builds, we just don't use it for the firmware. So, instead of the patch you recalled above, the tests/tcg machinery should be extended into something that can be reused for firmware. As an aside, orchestrating this multi-compiler part of the build is what the Makefiles will keep on handling for the foreseeable future. As an aside to the aside, tests/tcg is more than underdocumented and I forget everything about it 5 minutes after looking at it.
This is not something that I will be able to work on anytime soon. But still I don't think that going in the wrong direction is a good idea, even if temporarily.