You're not the only one saying this, but I just saw your post. Have you used any mobile GPUs recently? Current Mobile GPUs are more than capable of matching performance of desktop capabilities.
I have a 2 year old 17" Sager Laptop with an Nvidia GTX 880M. I can run nearly any game out there maxed out on highest settings(FXAA, AA, etc), no problem. No, I can't run it at 4k.. but my screen is only 1080p anyways, and in the last year or two there haven't really been any games that run below 30 FPS, except for a few where I couldn't turn on 16X AA, etc.
If you're attempting to play on a 4K monitor with 60+ FPS, then yeah, you probably shouldn't be using a laptop.
I think your confusing getting "good performance, relative to your monitor" with "getting the same performance as high end desktop GPUs." Actual mobile GPUs are cut down versions of their desktop counterparts. Yes I know there are actual GTX 980 GPUs in laptops which is still a far cry from 980Ti or TITAN X GPUs. You will also still pay substantially more for the same level of performance with a lower performance ceiling.