There are loads of odd gaps in magical ability. It's hard not to get the impression sometimes that life in the circle has stunted magical research - hence why circle mages are all shocked at Morrigan's shapechanging abilities. The circle does not teach that for reasons that should be obvious, and there's probably a lot of other things they keep secret.
What's odder to me is the gaps in northern mages' and Morrigan's abilities. Morrigan can become a cat, a wolf, a crow, and - circumstances depending - a spider, a swarm of locusts, and a dragon. She cannot, however, look like another human and her reasons are vague. Something along the lines of her "already being a human."
What's more, shapeshifting to other human forms - while something that demons readily do - is impossible even for those Tevinter magisters. For instance, Mae Tilani is a trans magister for whom no magical sex-reassignment surgery exists. When you ask the series one other trans character, Krem, about magical SRS he says "He wouldn't let any magic like that near his body."
The reason this is relevant is that it seems transmuting the body is incredibly dangerous for some unexplained reason. To produce and gestate a child, someone would have to carry it, and that would mean some kind of physical transmutation.
It's interesting that Thedas seems to lack so many of the basic forms of magic that are ordinary in other fantasy worlds. Time magic's limitations are featured heavily in DAI. There's no flying spell, no invisibility spell. Healing is rare. There is no teleportation, except slightly-bending-the-rule stuff like Fade Step, Stone's Throw, and Eluvians. It's like they deliberately set out to avoid the usual spells you find in other fantasy universes.