It wasn't always thus: On the 6502, which the early Apple II machines were built around, it was possible to access RAM at only a one- or two-cycle penalty compared to doing everything in registers and immediate values. This was only the case if you used zero-page memory without indexing, however, so you couldn't have a lot of stuff in RAM without incurring more speed penalties.
(Zero-page memory on the 6502 was the memory accessed via addresses with a high byte of 0x00. Since 6502 had sixteen-bit RAM addressing, this meant each page was 256 bytes large, so the zero-page was almost as good as having 256 single-byte registers.)
Doing 22kHz generation on a Macintosh is very close to the limit