Because most people can only afford one computer, and if you need a laptop, that will be your only option for gaming. Also it makes LAN parties 1000x more convenient.

If you want a laptop for comfort and convenience, gaming has to be really important to you to justify spending at least that much again on a desktop. It's something else to find money, space, and time to maintain.

A laptop is much easier to carry to a LAN party. (And yes, those still exist. I hosted one a few weeks ago, and it's still fun.)

Why not? An extra computer is a big investment just to play games.

And it's not just the investment in money; there's maintenance as well. You don't boot it as often as your daily machine so every time you do, there's bound to be an update waiting.

I quite enjoy playing games on my laptop. I was worried the track pad would be terrible for fps games, but it's fine (though I always rebind right click since that doesn't work well on a Mac trackpad). Why buy a separate machine if the laptop works well?