There are already PC series like Intel NUC or AIO desktops with mobile CPUs and they run pretty good but you won't install there any stronger graphics card.
The main issue with mobile CPUs is that most of them are in BGA version so soldered to the motherboard and it's expected that in some time all mobile processors will be only in BGA option. Most desktop users want to have a chance to replace processor or pick all components, not only use what PC manufacturer says is good ( usually it's good for office work but not always for everything else ).
There is also other thing. Mobile processors, except top and really expensive versions, are still low frequency. Turbo up to ~3.5GHz but for most time it works at 2-2.5GHz what is not always enough. Since Intel locked most mobile chips then it's not possible to overclock them. AMD doesn't count on mobile market for long years so I won't even mention them.