So I wouldn't say it's a stereotype. Just a marketing decision.
Yep, it's for all those who have never played their games before.
Just imagine if they chose the poster boy to be a skinny, little Bosmer with a battle axe bigger than his body? Those people would be saying, "Ahh, I don't really like the character they make you play as, not interested." Howard would then have to say, "oh, you don't have to, you can make your own character", and he would have to say that over and over and over again.... Plus, it doesn't make sense for a Bosmer because the game takes place in Skyrim.
It's all marketing.
You see with Morrowind, they could get away with showing a cool looking, but weird humanoid Dark Elf thing because TES was still unknown. In fact, I had never heard of it until my brother saw it at the store, read the back and thought he'd give it a try. He got it for a good deal too, so why not? When I first saw the cover I thought, "What the crap is that weird looking purple dude with the Kramer like hair? Nice armor though." Turned out I loved the game and so did he and many other people who spread the word. Morrowind is what made TES popular and when things become popular you got to start worrying about all the marketing even more.
Speaking of little Bosmers, I wonder what they're going to do about the poster boy for Valenwood? Probably going to be an attractive and young Bosmer woman with a bow, maybe some knives, you know that "rouge look". That would be my guess anyway.