Thanks for the reply.

One can expect to live a life at least 100-1000 times longer than those other poor people

.. when you say "can expect to", what do you mean? Do you mean "it is extremely likely that.."? That's the problem. If it was a sure deal, it would be logical to spend the money on it -- but in fact it's extremely uncertain, whereas the $50 being asked for by a group like Aravind Eye Hospital to directly fund a cataract operation is (close to) relieving significant suffering with a probability of 1.