Because twenty year-olds have assholes like tiny rubber bands, dicks like stalagmites, and a sense of wonder/naivety that older men eat up with a spoon. The older men want to be the captain of a virgin (like) vessel, sail the seven seas, going on adventures and swashbuckling their way to the lost treasure (the booty, arrgghhh).
The "booty" really being their "need" they can't get from a man their age or a partner. They may want to recapture their youth, want to "feel" young again, want to be desired by a young man to validate their sex appeal...you know, feel like they "still got it," maybe they want to have bragging rights around their friends because some definitely do it for "image," of course others view young men as good companions, and let's not forget that some young men seek out older.
These young guys admire the older man's experience, his worldliness, his masculinity, and/or his swagger/ je ne sais quoi. In addition, older guys tend to be gentlemen and younger are on their best behavior too with an older. I think it helps the planet; its the closest thing to acceptance, mentor-ship, and unconditional "like" going in our community right now.
Plus, FYI, FM2Y, nobody makes love to your mind like an older. The dirty talk alone, my god, you've never been talked to like this and that is just what he says. It starts at dinner, before he even touches you, and the whole time he's studying you, becoming cognoscente of your body language, which he'll exploit against you later. Once he touches you, it's like putting on your favorite sweater because it's like he knows you. He's been thinking about this and where he'd touch you for some time now, but you didn't know it. He could talk or not because he makes your body do things.
At the risk of ruining my pajama bottoms, I'll stop that, but older men are good lovers, pride themselves on it. They enjoy cracking a man's code and putting the puzzle of ecstasy together. They like introducing their partner to things they've experienced before, but will experience new with their partner.
The problem with other older guys is that they are hip to all of the older man's tricks. They've sailed the same sea and already had their own adventures. When he tries to read them at dinner, they thwart him; when he tries to touch them in places they've never been touched, they've already been touched there, and the whole time he is trying to exert his je ne sais quoi on them, they are trying to exert theirs on him. It becomes akin to two Jedi knights attempting to move each other.
Now, it can work, especially if an older guy appreciates dinner and dancing with an equal and enjoys the added challenge of making sex as ordinary extra ordinary, but I can't blame the nautically minded.