The problem with Mini ITX is that the PSU takes up too much room or there is a really weak PSU in there for size reasons. Why don't we see external PSUs for ITX PC?
Because you still have to individually cable power to different components and while you could have an external PSU, The cable from the PSU to the PC would be very thick as it would have to have a large number of cables. With Laptops, Everything gets power through the motherboard. Reason why the PSU's can be on the outside as they mostly convert the power from the wall to usable power in the laptop. PC's use a lot more power and are designed differently from laptops. PC PSU's do a lot more than Laptop PSU's.
You could always pull an EVGA and just stick in a PSU only powerful enough to run the exact number of top-of-the-line components that you can fit into the case in the first place.