Probably for a similar reason why they dont have Children in ES, for the rating and so people wouldn't abuse them in different ways.
It has nothing to do with the rating, Bethesda could make zombies have female physique while making them just as... lacking in certain areas as the distinctly masculine ones in Oblivion, lots of games pull this sort of thing and get a T rating (and usually due to violence rather than any semblance of partial nudity.) even Morrowind did it with Winged Twilights and female Khajiitor they could, maybe, give them clothes? Since dead people don't necessarily have to be naked, and I doubt that most necromancers are particularly interested in stripping the corpses they raise (unless they're like a certain alchemist in Oblivion, of course.) and I certainly doubt that once zombies are raised they have an impulse to remove every shred of clothing on their bodies.
Really, it stems from a far greater problem, which is the lack of variety in "monsters" in general. In most cases, an enemy of each type (that is to say, each specific type identified by name, within the "zombie" classification there may be some variety like standard zombies, headless zombies, and such.) uses the same generic model, something many RPGs do, as a result, they all look the same, there's no room for believable gender characteristics. The only instances where non-playable lifeforms seem to show signs of multiple sixes are in deer (with some of them having antlers and some of them lacking them, seeming to indicate male and female individuals) and goblins, and in the latter case, only the shamans have female models, even horses, despite some of them being addressed as female, seem to have a certain... male characteristic (no wonder so many people seem to think Shadowmere is male.) To address this, Bethesda would need to put some effort into giving your enemies a bit more visual variety, I'd say give each "monster" a couple different models, and then a little texture variety like in MMM, maybe some monster types, especially undead, should sometimes have a few pieces of armor the player can wear (though these might also just be part of the model rather than actual lootable items.)
Skeletons - Self-explanatory, they're skeletons.
The sixes in humans do have different skeletal strctures, though Bethesda probably (rightly) assumes most players wouldn't be familiar with these differences. In any case, with skeletons it's not a huge concern, a far greater problem is the fact that all skeletons look human, although they might be able to be forced to pass for elven skeletons as well, they won't work for Orcs, Khajiit or Argonians, if we ever get a game set in Elsweyr or Argonia, things will look really silly if Bethesda still uses only human skeletons, either Bethesda would have to do multiple races of skeletons, or Bethesda would have to do something unorthodox like making the generic "skeleton" not appear human.
Obviously, the necromancers that raise the skeletons chose the strongest, manliest frames they could find because they were meant to do battle.
I doubt that sort of thing makes much difference when you're dealing with skeletons, since they have no muscles or flesh, their strength would be determined by the magic animating them.