[awillen]

The straightforward and cynical answer is that the people funding these things have basically unlimited money, at least for practical purposes, but as with everyone alive, they have limited time. Therefore it's a perfectly rational trade to give up even a substantial percent of their money in exchange for even the possibility of more time (especially because if you have a lot of money, more time means you can turn it into even more money).

But also, while a lot of the headlines are about living longer, the research is largely about treating aging as a disease, or at least a cause of diseases like Alzheimer's, etc. Even if you don't actually extend human lifespan, you keep people healthier much longer, which among other things has enormous economic value (people don't required as much expensive medical treatment and can continue to be productive for longer).