There are many many reasons and most of them to do with getting or hoping to get fun and sexual advantage for themselves..
I am not going to be able to do justice to the subject here as I don't want to type reams so please excuse the brevity
1 reason is the perception younger men have that older women are desperate - they believe that older women don't get much male attention and this makes them easy pickings
2 is that they believe they are doing the woman a favour and that she will be soooo flattered she will easily agree to sexual favours
3) is the idea younger men have that THEY are ultra hot compared to the older woman so there's power in that, again the lady might well feel she has to give more to please this self-proclaimed hot guy
4) the younger guy has the perception that older women, (perhaps because of their perceived desperation) are happy to give sex/favours without expecting to date, be taken out etc.. therefore easy sexual favours with little effort on the guys part..
5) the younger guy has the perception (not necessarily erroneously) that older women are more relaxed in bed, more experienced - some guys like the idea of being 'taught' while benefitting from free sex etc
6) Again buying into the perception of desperation and experience - younger men can perceive that older women are more willing to do wilder things in bed.
7) Some younger, (and older! ) men can buy into the idea that generally speaking older women tend to be more submissive when it comes to men, generally we were taught to be nurturing, caring, and to please men.. that can be a bonus if there's a perception that older women will be more manipulatable, more giving, less likely to say no, less assertive and perhaps (feeling self-conscious about their age) more likely to say yes
Of course that's not to say that some younger men don't genuinely have a thing for much older women.. some do.. but by far the vast majority have no idea how many young guys pester older women and are looking to get pics/sextalk or something else sexual from the older woman based on the perceptions above