iMacs are usually laptops in all but name. To hit the same thinness with a giant heat-producing screen, most of the parts are their mobile equivalents.
Edit: on slightly closer inspection, this isn't true for the 27" models!
The 27" models have traditionally used off the shelf socketed CPUs though, which is unusual in AIOs. Regular 27" models use desktop i9s and the Pro uses a desktop Xeon. Both can technically have their CPUs upgraded even if opening them is a pain.
Thank god for them being thin, I actually had to throw away my 2013 iMac because it was just unbearable reading my emails with x-ray vision and noticing just how thick it is.
What is hilarious is that the thin imac weighs the same 20lbs as the one that was a few inches thicker and had better cooling and user replaceable parts.
Edit: on slightly closer inspection, this isn't true for the 27" models!