While I realize that in the real world, there are different heights, of different people, including adults being shorter than teens in the real world, and there are couples over a foot apart in height difference. I feel that height would lose the distinction between adults and teens. That being said, if I made two Sims the same age, a small female who's 4'11", and her 5'11" boyfriend, to me, it will look like a normal adult Sim, taking advantage of a young teenager. I realize that my 5'0" self, has encountered an average 5'10" male (yes, even an average 5'10" male looks tall next to me!), in real life, but in the Sims, where everyone knows the distinct difference between a teen and a YA/A, and everyone is in uniform height, it won't look appropriate.
That's not the primary reason, that's the secondary one. The primary reason, as I said, is that it would mess up the animation, and cause severe problems.
EDIT: What about normal, non RPG types of Vacation? I'd like to see just staying at a hotel, and not having to go through all those travel levels.