If I don't build it myself and can exploit myself, I get nothing and somebody else gets everything, so why shouldn't I just shut up completely? If I contribute something to the design of nuclear power plants, that the nuclear plant people would never come up with because people from my field, whatever that is, don't look at their stuff, then I obviously can't build my own nuclear power plant to compete with them.
The only way to ensure that people have an incentive to invest their time into things outside of the stuff they do to get money is by giving them patents.
Another reason patents are nice is that it's that you get something for your actual contribution. This means that it offers particularly skilled people who aren't rich a chance to actually build a company and have something real.