Apple reduces friction to the point where even my mom could upgrade the RAM on her iMac, and it can do this because it controls everything that goes in that box.
Yes, but you can *only* replace the RAM in an iMac. If you want to replace the SSD/hard disk or CPU, as a true "gamer" might want to do, then you're going to have all sorts of fun pulling it apart (a specialist pizza cutter to remove the adhesive behind that pretty glass screen, specialist screw drivers to un/refasten everything inside, depending on the model a specialist temperature sensor/SATA dongle to stick on the new SSD/hard disk, and new adhesive strips to stick everything together). And you'll never be able to upgrade the "video card" it's integrated into the motherboard!
Now try to game with your shiny new and upgraded iMac, whose warranty you've just invalidated... most games on Steam are Windows-only, especially AAA games, and of those that are "OSX compatible" many don't perform well on high resolution Retina displays.
Just buy an Alienware gaming machine if you're too delicate to build one from scratch.