Why would they do this? I could understand using a non-GPL make because they hate it, but using an ancient GNU make is just handicapping your users for no gain.
Apple has restrictions about what software on the system you can modify as a user and how, in the name of security. GPL 3 is unfriendly to such restrictions. Whether what Apple is doing on the Mac specifically violates GPL is, well, a matter of debate that has never been tested in court, but Apple thinks there's at least some risk there, and that the risk isn't worth taking.
This is also why ZSH is now the default shell on the Mac. ZSH never switched to GPL V3, so it was either that, remaining on some god-awful old Bash version, or making their own.
Don't ask me. One could argue that this is the version they included in some early version and have kept for compatibility reasons, but that doesn't make(sic!) sense.
No. Just a really old version of GNU Make