The original Xbox had a tray because it was basically a PC, and had a standard IDE CD-ROM with some minor changes. They weren't standing apart, they were following the new standard of the time, PCs, and it was probably more to do with cost savings by using common parts. They also had a standard IDE HDD. Even their weird proprietary controller port they used on the original Xbox is just USB! Its the same wires, they just screwed up the pinout. you can replace one of those weird controller ports with a normal female USB and then plug all sorts of USB devices into the Xbox and they just work.

I only single out the Xbox because I've taken them apart, I imagine the PS2 is similar. At least PS2 didn't intentionally mess up their USB ports.