Says who? Where are you getting this information? Tell that to the HUGE numbers of people that listen to headphones with a DAP. Have you tried these "many high end full sized headphones" on all DAPs to see that it is true? There are also many that can. And there are many IEMs that can't be driven as well from a desktop rig. That has nothing to do with the form factor. It comes back to those pesky numbers, and synergy.
You don't need 45W, or a device labeled "desktop usage only" to drive high end gear properly. Sure, it's nice to have on tap, but that doesn't (necessarily) make it sound better. There is a point where the term "scales well with power" no longer applies. Most head gear lists input power or rated power. That's it, it doesn't take MORE power...
Forcing more power than that will (most likely) lead to "letting the smoke out of them". That isn't to say that there's no desktop gear that sounds better than DAPs, and vice versa. But you can't simply make the blanket statement that headphones CAN'T be driven as well on DAPs. This just isn't true. I can name a ton that the manufacturers would argue (pretty vehemently) that they can.
With good gear, you only need to have enough power to cover voltage swings, impedance spikes, etc... for them to be driven properly. What you might be thinking of is synergy, and the type of device doesn't play as big a role in that. It is all about implementation, tuning, output impedance, etc... Most contemporary DAPs ARE desktop gear in a smaller package, and with a battery rather than a plugin (that and they are AIO).
Let us agree to disagree on this subject. There is a reason that the manufacturers give these numbers, and while some like to conflate or deflate them, they are given for a reason. It isn't just to look perty' on paper.
We could both come back with our version of information that makes sense, and in the end, we would probably say something "not so nice" to each other, and I don't want to do that because I value (and have for a long time) our conversations.
Peace be with you!