Carnegie, for example, decided that He Really Liked Libraries. So he built a lot of them.
Paul Allen decided that we were wasting everyone's time redoing basic science experiments, so he did a lot of them and put them all online for free forever.
Brian Acton and Jan Koum decided that free, secure messaging should be available to everyone, so they sunk $500M into Open Whisper Systems so it can be a nonprofit with a warchest.
In years past, people have made similar decicions like "there should be a really good hospital in this town, so i am building one", or whatnot.