I think it's mainly because there is no effort involved. There's two ways to get points on imgflip: put effort into memes and gain the respect of the community, or just straight out ask for upvotes. Everyone who gets mad at upvote beggars is obviously not one themselves, so it upsets them that people are getting points with no effort. If you go back in time, upvote begging memes actually used to be good, but now they've lost their touch.
Tl;dr: because there's no effort involved.
I hope this answered your question.