Oh, just try building the Xilinx ISE libraries with ghdl in -2008 mode. What makes no sense to me is that analysis is really the language-version-dependent part, not elaboration, unless there are obscure linkage/binding things that are so different in 2008 from previous that they can't manage it.
I'll take a look. I admit I never bothered trying to simulate with ISE libraries with GHDL. Not sure what VHDL-93 construct would "break" using -2008 rules. One thing I do know is the (in)famous use of those old Synopsys libraries like the std_logic_arith stuff. Those are not standard IEEE libraries, so GHDL supports them only if you pass a specific flag on the command-line.
Possibly Xilinx otherwise used functions from IEEE libraries that have differences in VHDL-2008, or possibly they didn't quite use them properly. Not to be too harsh, but pretty much all Xilinx VHDL code I'ven see was pretty horrible. IMHO.
As to elaboration, I'm no expert of GHDL's internals, but I know it uses a pre-compiled version of the standard libraries for each supported version of the VHDL standard, so that's probably one reason you can't mix standards at the elaboration step.
But when your projects have a hundred source files, and the projects include test benches and simulation configurations for all of the subentities, then having a GUI that shows compile status, compilation order and all of that is, you know, really nice. And the simulation configurations are quite handy. An entity or a configuration has different top-level generics? Cool, just set up different simulation configurations with the different generics and off you go.
I can understand that. Probably any IDE properly set up for VHDL/GHDL would do the trick, though.
I should say that I haven't tried to build it on macOS in quite some time. Maybe it's easier now. But, if it was easy, why don't they do it?
OK, apparently you managed to do it. Well, building binaries for different platforms takes resources. I'm sure it all depends on which maintainers are currently active, what kind of machines they have, their priorities, etc. I personally do not have any MacOS machine (real or virtual), for instance, so I wouldn't be able to build MacOS binaries - and even less test them.