I believe they do not "do what they want" at all - or they would have done so.
- They would have switched to M* (whatever) day one (last November)
- They would have shown off the new iMac with a processor that is suitable for a desktop instead of a mobile one
- They would offer more RAM for the M1 SoC
- ... and so on
My guess is that there are / were difficulties to overcome with "the next level". Which is why they chose the M1 SoC for the iMac. A choice I find very strange. Why put a processor / SoC that needs so little energy into a machine that has no energy constraints whatsoever?
I guess that even though there are a lot of rumours, they simply are not ready yet to go the next step.