This is pretty much false. Just go look at all the parts you can buy and all the different versions of those parts. Mobos, memory, processors, cases, fans, liquid cooling, video cards, sound cards, and so on. That isn't a market that is catering to people who just want a laptop. In fact, since the introduction of Gaming Laptops, that market has grown. Even with Android games that market has grown.
The reason? I am not an engineer, but I can plug things in. My desktop can be fixed in a day. I do not have the ability to fix a laptop and I don't trust anyone to do it for me. I used to be okay with paying others to fix my PC, until one time they couldn't. So I found my own solution, took me 30 minutes online and I had my answer. Mobo was fried, it was clear as day and the people whom owned a business couldn't figure it out. Went down to Microcenter, got a new mobo, and was gaming in a few hours. A big part of why I exclusively play games on PC is the control I have over it.
When the time comes that a laptop can give me the same level of control at my admittedly low skill level, then I'll be first in line to pick one up. Desktops do have their downsides, but if you want to get into PC gaming, they're still the best option. Get a laptop if you want to occasionally take a break from a console to play a couple games on PC.