I think the main reason is that most people doing this sort of work don't want an all-in-one, but I suspect thermals play a role as well. The pros of the iMac are vastly outweighed by the cons for performance and professional use.
What are the main pros of the iMac?
1. Easy to setup.
2. Fewer connectors.
3. Looks really nice.
None of these matter to someone using their computer professionally for anything remotely technical. Personally, the combo I have wanted since I've stopped using a laptop as my primary machine is something like this Studio setup.
People who need a lot of power tend to use more than one monitor as well and, in my experiences, quality monitors tend to outlive the computers they're attached to.
The iMac was originally a consumer/non-tech product. It seems to be returning it those roots, which is nice. Squeezing it into the "Pro" world was always a mismatch. (I have a buddy in the Pro camp who is super salty about this, but he is unusually concerned about the number of cables coming from his desk, lol. He really wants a bigger iMac with the cool power + ethernet cable.)
If Apple finally kills off the awkward 13" MBP (which I suspect is coming when the M2 is released), their Mac line will be really clear for the first time in a long time. (I personally hope they drop the "Air" moniker and just call the new laptops "MacBooks." A 12" and 14" would really round things out nicely.)