I suspect that the cost/benefit isn't there. Those who need the "biggest Ultra" will be happy with the previous generation or so, and so they'll refresh that on a 2 or 3 year cycle.
M3 Ultra, 3.3k single core 27k multicore on Geekbench. https://browser.geekbench.com/v6/cpu/16959045
M5 Max, 4.3k single core 29k multicore https://browser.geekbench.com/v6/cpu/16956481