Most laptops/tablets use two core (e.g. Intel max. 15W) processors without discrete graphics card. If the laptop has a four core (e.g. Intel 45W) processor, then it most probably has a discrete GPU also.
Then again, someone might want to use their top-of-the-range gaming laptop with a desktop graphics card in a separate enclosure just for the heck of it or for some other obscure reason, there are all sorts of people and they can do what they please. I am not offended , hey, go for it.
There are some laptops with four core AMD (APU) mobile processor without a discrete graphics card, but these aren't anymore powerful than Intel two core mobile processors.
There probably are exceptions to the above, but please don't hang me if you do find one or several I didn't check all the laptops/tablets available on the market.
However there are some Small Form Factor (SFF) PC's that have a relatively powerful processor.
Have to though remember that this is a future market. There aren't that many devices on the market that actually support eGPU's.
Anyways, even a small PCB can have sufficient power connections for external power bricks to run any graphics card.
Why I am not that interested about using a ATX/SFX PSU, is that such a PSU utilises only part of its power connectors with an eGPU and significant part of the connections are wasted. Unless you use them for example USB charging.
It would be nicer to have a smarter (and smaller) power supply solution.