Or perhaps they bought them because they had no other choice when choosing a reasonably fast desktop machine compatible with their software?
Seriously, the argument "oh, this feature must be popular because the whole machine is popular" really doesn't hold water in complex machines like computers, phones or cars.
iMacs sold better even when the "cheese graters" were available, so yes.
Apple never had a usable entry level desktop computer tower. Something with high performance on consumer class CPUs, rather than Xeon and ECC ram. The Mac Mini was always crippled to keep it from competing with the iMac and cheese graters among people who want something better performing.
The iMac is popular because it's the best performance to price ratio. Not because the form factor is any good. For the longest time the entry level MBP (and unibody Macbook before then) was also Apple's best selling laptop and they only recently cut it out from their line-up and replaced it with the Air as their entry level offering. The Air will also exist for as long as they keep selling the current Macbook at those prices because most people are not willing to spend 1449 euros on a machine that barely performs better.
For you maybe. Many of us (millions, judging from the sales numbers) appreciate the thin-ness, even if it's for a desktop machine.
I had to drag my old non-thin iMac several times to different locations, and it was no joy. Nor do I need a behemoth on my desktop.