I think a big part of it is the idea of fundraising itself - any fundraising. The glory always goes to the person who did the 'fundraising' and it seems to slip a lot of people's notice that it was others who actually stumped up the money - some of them quite possibly in a parlous financial position, but made to feel awkward or mean by the 'fundraiser if they didn't give what they struggled to afford. Even more so if the 'fundraiser' is very wealthy themselves and thinking everybody could at least chuck in 50 quid, when for many, even £5 is more than they have to spare. Of course, plenty of fundraisers do it passively, just holding a bucket in the high street and letting people approach them if they choose to; but when it's somebody you're related to or whom you see every day at work bounding over to you with an enthusiastic "Oh, I know that YOU will sponsor me, won't you?!". Especially when it's still near the top of the form and they, their spouse, their mum etc have sponsored/pledged £100 each and you know you'll look mean to everybody if you're the first to 'only' give a tenner.
Also, I agree with the PP who said some of these people are simply deluded or a bit dim. If a charity tells you that you must raise at least £1,000 to trek Machu Pichu, they think the charity is setting them quite a big target because, naturally, they want to encourage people to raise a significant amount - it doesn't even occur to them that all or most of the minimum is the cost of their amazing holiday/experience and only then, once they've achieved that minimum in sponsorship, does any excess actually start to go to the charity as a net receipt. As some PPs have said, if your motives really are charitable, the very least you should do is give the minimum amount (ideally more) to cover the cost of your holiday, before you start asking people for sponsorship which will actually 100% go to the charity. Even then, though, you'd still get some people exploiting that principle and grandstanding: "I've already given £1,000, myself, so don't tell me that you can't dig deep fir a great cause too." Yes: you've bought yourself a lovely holiday and now you're using that rather mundane fact to try to make me guilty if I don't give a wedge to YOUR favourite charity.
Actually, I wonder how many people choose the charity that's close to their heart first and THEN see what kind of opportunities their chosen charity is offering. Call me a cynic, but I bet a fair few first decide what holiday or experience they want to do, then see which charities are offering it, then see which has the nicest terms (longest trip, smallest minimum amount to raise etc - or just which one has a place left) and only then suddenly start urging everybody to 'get behind them and support this amazing charity that means so very much to them personally".