I don't think the last digit would be really stable between 1 min after turn on and fully warmed up (1 hour). So the resolution is of limited value.
The benchtop 6 digit meters nearly all use a ovenized reference. This is not really practical in the cramped space.
With the compact form the temperature rise is also larger than with a more spread out bench meter.
I've never looked at benchtop DMM teardowns, but from what little I know, at least some of them use a fan. Trying to use a fan in a portable DMM would likely be silly, due to size constraints and a larger draw on the battery. Also, for a non-battery powered device, and where a fan can be added to the design if necessary, there's no limit for the designer in choosing exactly the components necessary to get that increased accuracy, even if they're power hungry and generate a lot of heat.
And then you have market forces. Everyone needs a scope and handheld DMM, but not everyone needs a precison bench DMM, therefore sales volumes would be lower, less agressive competition for that market space etc.
For me personally, if I was extremely limited in what items I could have for my workbench, I'd of course need a power supply, followed by a scope and then a high precision benchtop DMM. Sure, for basic measurements I could use either a handheld or a benchtop, but for my use I have no need for the portability of a handheld. ALL my measurements are done on my workbench, and some require the high accuracy not typically found in handhelds.
Of course I realize I'm probably not in the norm, as so many members here do seem to think that a portable DMM is a necessity. Now if I needed a DMM for checking the house wiring, fixing appliances or other items that aren't practical to move onto or next to my workbench, then the workbench DMM would be impractical and a portable DMM would be necessary.
Anyways, I agree there's a much larger market for portable DMMs, but I disagree that everyone needs a handheld DMM. Rather most need, or at least prefer to have a handheld as their only DMM, if limited to only one.