Why do billionaires spend their money on space when they could spend it on the planet?

For example, Elon Musk is spending billions on space to send people to Mars. If that money was spent on the planet, he could do a lot of good. Like build solar powered energy stations or build water filtration plants. There are billionaires who are spending money on space but not as much as Musk is. People have said that there is no point in spending money on the planet because it's fucked anyway, so they rather spend it on space. Well, what's the point of spending it on space if the planet is fucked? If the planet is destroyed it's all over for human life because humans can't live in space.