And you will be able to upgrade a desktop computer. You could at some point swap the GPU or buy another stick of RAM for $60, whereas most things are soldered in laptops nowadays. Oftentimes they even solder the RAM to move it closer to the CPU and make the laptop a bit cheaper since it now requires less mechanical brackets/parts.
Also a laptop will almost never get the same performance because it’s more difficult to get all the heat out and it’ll switch to a lower clockrate once all the heat builds up in that small form-factor.
But it can be worth it if you need one device that can do both gaming and be carried around. Desktop replacements are quite popular. But they come with exactly those downsides. And it may be or might not be cheaper than buying one ultrabook plus a pc that’s tailored to gaming. It’s always a compromise, though.