Why do people say they admire certain types of people but then go out of their way to date or marry someone who is the opposite?

I known couples who look at other couples and think they are the idea couple they wish they could be but when you look at the person they date or marry they always seem to gravitate to someone who is quite unlike the very people they admire.
Is it they subconciously believe it is something that is not attainable for themselves?
Or is it they believe they can’t live up to what they need to be to the other person in order to have this idea relationship?
Or does love make them blind and forget what they were really looking for and they don’t realize till they are in deep?
Putting aside low self esteem as a possible reason or thinking that they are simply not worthy of being happy: what other reasons can there be?