Laptops are all well and good, and modern high end laptops can be quite good for gaming purposes, but there's a trade-off: You're still gaming on a laptop. No matter how nice that laptop might be, it's still a mobile device with size and usability limitations. All of those can be overcome mind you, but if you're hooking up your laptop to a giant monitor, surround sound system, keyboard+mouse, etc, you're essentially using it like you would a desktop - if that's your "gaming setup", you can build a system with a lot more punch for your dollar by just starting with a desktop in the first place.
The thing about laptops is that the technology that goes inside them carries a cost premium - a lot of the components that go into a full-sized computer are nowhere near as small as we can actually make them, that's why we have laptops that can run modern high-end graphical applications while taking up a fraction of the space; miniaturization jacks up the price though, which is why traditional desktop components aren't "as small as we can actually make them". Then there's the issue of modularity - with a laptop there is really only so much wiggle room for "expansion" after the fact; depending on the design it may literally be impossible to upgrade.
If you need to have a mobile computer and also want to be able to game while you're out and about, then yes, purchasing a gaming laptop makes sense for you. If you're just looking for a computer to use in your gaming "sanctum sanctorum" and nowhere else, turning a laptop into a desktop fixture is counter-intuitive and not the most efficient use of your money.