There's no blanket answer for this. First of all, being pretty and educated doesn't automatically mean that it's easy to find somebody who's right for you.
I have friends who are absolutely gorgeous and intelligent but they're also super shy, have trust issues or the like. And then there are also beautiful women who men see and think "she's out of my league", so they don't get approached, and some think they're just good for a purely physical relationship and can't commit etc.
Another point is that some people have very specific wants and needs and so they turn to dating sites because it's easier to find a match than to aimlessly be looking in your surroundings. Other people just like the convenience of dating sites.
And these are only a few of the reasons. People are complex, they all have their own thought processes.