In the context of college students, I agree with Alexander Woo's explanation. By the way, the best and the brightest often place out of calculus (that's the case at Yale, and I imagine it's not that much different at Berkeley), so the percentages of weak students at best schools aren't as dire as you might think.
Concerning the last question,
"Why isn't discrete mathematics offered to high school students without calculus background?"
Not only is that possible, but it had been the norm in the past within the "New Math" curriculum, when everyone had to learn about sets and functions in high school. This ended in a PR disaster and a huge backlash against mathematics, because generations of students were lost and got turned off by mathematics for life; some of them later became politicians who decide on our funding. Consequently, it was abandoned. (Apparently, calculus in HS was introduced as a part of the same package and survived.)
I'd be interested to know if there are any high school – college partnerships that offer discrete mathematics to H.S. students with strong analytical skills, and how do they handle the prerequisites question.