The 5K iMac GPU isnt really capable even though spec-wise it could be (in 1440p).
Please note that I said 1080p. Not 1440p, not 4K and certainly not 5K. Just 1920x1080p. The GPU in the Retina iMacs, specifically the higher end versions, are pretty decent. Not top-of-the-line 300W monsters, but conceivably fast enough to run a AAA game at 1080p @ 60Hz.
That is because of thermal throttling when you start using it.
The new 2015 model has been modified such that this is not an issue - at least according to reviews. The 2014 versions did have throttling, but it shouldn't have prevented them playing games well. In fact in the reviews I've seen the old M295X model outperforms the newer M395X model - despite thermal throttling.
I just got an iMac with the M395X at work & I'm looking forward to stretching its legs.
So no, Macs are not and have never been made for people that want to kick back and play some games in between working sessions.
That's cobblers - the iMac line has never been intended primarily for work. Each generation was explicitly designed for & marketed to consumers. Various Macs have shipped with games as part of their Apple software bundle (Nanosaur in the early iMacs like my Rev C., Actua Soccer with a PowerMac 6500... etc) because Apple do know their consumers want to play some games, both AAA & otherwise. They just don't spec. their lineup like gaming PCs because that's also not their primary purpose.