I'm not super familiar with arcade hardware, but it's always fascinated me how the Z80 and the 6502 dominated the market for reasonably-priced CPUs for about a decade. You'd just use one for everything. The 68k would eventually come into the picture, but even into the 1990s you still had a major console like the Super Nintendo using a 65816, which is essentially a 6502 with 16-bit extensions.
Games were getting really big in the 80s, but it took a long time for hardware R&D to kick into gear.