Earlier quoted context omitted.
It makes a lot more sense if you look at the iFixit teardown. https://www.ifixit.com/Teardown/iMac+M1+24-Inch+Teardown/142...
Does it though? The iMac is basically the same as the M4 iPad Pro, and the iPad Pro doesn't have a chin.
Cooling seems like it might be a factor here. The iMac's display is probably going to be run at a brighter (and thus hotter) setting AND it's more likely to be used to do things that require high load for extended periods of time, so putting it in its own space probably helps.