I have assumed that if a " contestant " goes until the end of the show that there is more $ in it for them and that is why Luke continues with the show. He certainly could care less about Kate.
She is a sweet kid and he is just an ass.
A.J. just doesn't want to eat alone any more. That seems to be his only goal. Also an ass.
They would never want older couples on this show because there is no "drama" there.
Older people have been around the block and won't take the bad behavior ect.that comes with this territory.