There will be a portion of celebrities that do it for their ego or because a public relations person has told them to improve their image. However there are plenty of them who do it for purely altruistic reasons and probably do more for charity than the contributions of millions of others collectively. It shouldn't really matter to you anyway, as a single extra dollar is going to good use whether it's for vain reasons or not. I think it's a great thing how people like Bill Gates are pledging to donate billions after they die and wish others would copy that model - even better when they start early. Media exposure for the people at the top is probably the last reason, because they've already had more than enough.
Charity is good quality. On earth, there are every kind of people and five fingers have not same size and shape . They have fear of God as they are investing more money on that . There are also lot of people who want to get fame by charity and for this purpose they made YouTube channel to show off but that is not good thing for the society. On that other hand , poor people are getting money and they are able to buy food for living, that is also a point. We have to the think through all angles then we will say that every person is doing for what he is doing and we can't stop that .In that time, almost 90 percent of the want to get fame and they are using different techniques and that is one of this.