Mind you, even as a child I was unable to sit cross-legged (without holding my ankles and arching my back) and found floor sitting immensely awkward and uncomfortable.
My impression is that amongst the different "races" we have different ranges of body shapes; I am relatively convinced that this makes some actions and positions more difficult. Maybe squatting is such a movement.
Interestingly, the squat is a very similar problem, but in the opposite direction -- your ankles need to bend the other way. Lack of range of motion makes it so that you can't position your weight forward enough to stay on your feet. There is also the compounded problem of needing the flexibility in your hips.
Finally, sitting cross legged is also related to ankle range of motion -- but in a kind of diagonal way. To sit comfortably cross legged you have to be able to put the tops of your feet flat on the floor. Of course, this requires flexibility in your glutes as well, but if you don't have the range of motion in your ankles, your feet/knees are always going to hurt. A full lotus position is actually easier on your joints, but requires a lot more flexibility.
Anyway, like I said, there is definitely a range of starting places for flexibility for each person, but no matter where you start you can get there (barring injury, or other kind of unusual mechanical problem). Strangely, I find seiza to be the easiest of the three positions even though I initially found it to be the hardest (after you fix the range of motion problem in the ankles, there isn't really any flexibility you need).