Because it looks like USPS may have made that one of the new requirements in the 2019 DMM changes. (may not be there now and maybe added in a mid-year update if not? or may be internal policy that doesn't fall under DMM)
It looks like transmittal of dimensions for creation of all Priority, Express, and Parcel Select labels is now required.(still unsure - haven't had time to chase down the final rulings - only saw the proposals), probably because of the changes where dimensional weight considerations will apply to all of them starting 6/23/19
First Class Mail may not have that requirement (again, unsure and haven't hunted down new specs), but maybe eBay programmers got lazy or misread the new specs and just went with require dimensions for everything.
What doesn't PayPal, Amazon, the classic label flow, and others require it? Because they are old interfaces, and likely USPS didn't require reworking the old interfaces - just any new PC Postage interfaces created.
That is all speculation based on observation, that an eBay blue said something about it somewhere here a while back, and a couple of Federal Register documents or USPS bulletins I stumbles across couple of weeks ago.
TLDNR; I believe USPS requires it now.
