The problem is that each system has its own unique architecture. Beyond just getting a CPU and GPU for each system, you need the right kind of memory, bus, and blah blah. I guess there's the thing
JW talked about, where you rip the guts out of each system and slap it into a PC case. That's definitely doable, of course it costs more than each of the systems individually.
Sony managed to get the PS1 architecture down to a single chip. It would be awesome if the same could be done for the XBox, PS2, and maybe a handful of other systems, so you just have a mainboard with four or so FPGA sockets, and enough memory and a powerful enough GPU to run one at a time.
Come to think of it, you can do a lot of neat things with FPGAs these days. You could probably get an NES, SNES, Genesis, TG16, N64, and PS1 onto a single chip and install it on an FPGA card. The emulation work has already been done, so you'd just need a good interface to bring them all together. I shall leave that as an exercise for the reader.
