I don't find them boring. They're simply less foreign to our real-world frame of reference. Doesn't make them any less interesting to me.
They're not boring.
Just not as interesting as the Dunmer.
Nailed it.
It is in some ways kind of unfair. Morrowind felt like someone really truly tried to make them a real culture with vulnerabilities and strengths, white, grey and black elements to the culture that made some things despicable and others impressive. Interesting and very believable conflicts between various parts of their culture and a rather interesting history that you can see in the people. I think they just got more attention. Then you have to realize they feel more alien to us because of certain things injected rather smoothly into their culture like giant mushroom and crab houses with giant insects and weird domesticated animals. The Dunmer merely have more detail going for them. So we know more about them and that's why they seem more interesting to some of us.
The Humans are very identifiable for us on a superficial level and that is all that matters to most people. If you look at their cultures and ignore the obvious superficial oddities you see they all are actually quite realized and interesting cultures.
Humans are not boring, they are familiar. Familiar means a penchant for thinking they are just like something you have seen before and thus nothing special and surely there is very little of interest below the surface because it will just be like the others.