Actually I think this is a really good question. It's kind of more historical though I guess (and I don't know the historical answer either, but from building products perspective I think this is probably the reason/s).
75V DC is a cut-off value for safety testing on products (OSHA / UL requirements in North America | LVD in Europe). That's a good reason to not use 75V DC.
I think the real answer is probably pretty multi-disciplinary on the economics of producing batteries.
I have some family friends who own battery factories overseas and know a bunch about LiPos from a producing consumer goods perspective and safety issues but I'm no expert on the chemistry of batteries.
For some reason the chemistry of batteries appears to be really easy to produce in 3V & 5V regions. My guess is that the batteries got commercialized first at 3V / 5V and then the ICs were built in order to use the cheapest and readily available batteries.
(Your dollar store AAA battery is 1.5V -- add in 2, and now you are at 3V. So if I'm building semiconductors do I require my end user to go fabricate a custom 4V battery or design the whole thing around the fact they can go to the dollar store and power up my gear for $0.30 of batteries?).
Whole point of hardware is to sell it.
Same thing on the regulatory side: FCC creates new rules for how you can use RF airways, and viola your local RF semiconductor company creates a new chip to maximize usage of the new rules. First comes the regulation and next comes the hardware.