Because it's not. When you grow up you'll figure out that the sex typically falls in line when you click with someone.
...or the way I think of it, good sex is a symptom of a good relationship, not a prerequisite.
Likewise, if the sex is bad, that's often a symptom that there are other problems in the relationship. So stating up front that you want good sex is kinda like saying you want to not run a fever and oh, by the way, you want to be physically healthy.
Of course if there's no chemistry, no sensual connection, that should be enough to steer two people away from each other.