Things to remember about the orcs, they used to be Elves before being cursed by Malacath for not being ugly enough when they sought his aid. I imagine some a lot of them would still want to return to that beauty.
They're still too petite. And that still doesn't explain why only the women would "want to return to that beauty". It doesn't even work as a handwave - it's just the devs going "women can't look un-pretty lol"
Also it wasn't as much a direct curse as it was a metaphysical reaction to the shame and revulsion to the abasement of their god by Boethiah (S/he ate Trinimac and shat him out, which turned him
into Malacath), and the resulting disgust the other Altmer felt for them and the ostracism for Altmeri society as they became the Pariah Folk.
...Yeah, Orc appearance and nature is basically a physical manifestation of their collective bad self image.
As for all humans looking alike, that's kind of the point of humans, they almost always descend from Nordic Stock.
Most humans on Tamriel are not descended from the Atmorans/Proto-Nords, but are indigenous to the continent. The "out of Atmora" theory was rebuffed, it just didn't make any sense when you put the rest of the lore together. Basically, if all human (Redguard excepted) were descended from the Atmoran refugees/returners (because they came from Tamriel to begin with as well), they somehow managed to spread to all provinces except Morrowind, Summerset Isle and Valenwood (Yes, there are/were humans in Argonia as well) in a matter of a few years, develop a history and relations with the Elves and other creatures that run far further back than that, and develop a vast amount of cultures that is completely different from that of the Nords. And all from crews of a small number of ships.
Nah, there were human peoples there all along. A lot of them interbred with the Nordly arrivals over time, though, like the already Elf-interbred Bretons and the Colovians of Cyrodiil, mixing the their cultures.
edit: slightly Ninja'd. That what you get for pausing before you're finished.