The R&D needed to invent an entirely new CPU architecture isn't worth it for a game console, so they all use some off-the-shelf architecture - in this case, PPC

The R&D needed to invent an entirely new CPU architecture isn't worth it for a game console, so they all use some off-the-shelf architecture - in this case, PPC, which is a 32-bit architecture like most other PC/workstation architectures of its vintage. There are relatively few architectures which have address sizes that are not a power of two, mostly because this means an array of addresses requires wasteful padding for alignment. And for 32-bit architectures, even at the time it was realistic for high end workstations to have 4G of physical RAM, so there was a good reason to support this at the architectural level. Removing this for a game console after it was developed would have been more expensive than leaving it in.