Well, the disc trays were created on machines intended to lie down... the standing up part was simply a niche feature.... But the trays were never reinforced to be used at that angle.

But most PS3, Wii, Wii U, Xbox one and later are fine because they have no disc trays.

Makes sense when you think about, the plastics trays are flimsy in most cases. Not like Computer based Disc trays which are solid plastic and made for any orientation