I think because they have experience of all the ups and downs of life - and the disillusionment that can occur in love as with other things in life. People do claim that when they met it was love at first site and who are we to argue. But it is a rare occurrence and most of the time it is a deep infatuation that is mistaken for love and things can turn sour quite quickly.
Because we are older it doesn't mean that we have forgotten what it is like to be young, it's just that we have become less impressed because of the reality of life sinking in. Parents will say things like love at first site is not real, mostly to protect their youngsters from being hurt and disillusioned too early.