An idea is a starting point. That's it. It's like loading the first page into the typewriter for a novel -- the real work has yet to be done, and success is not guaranteed. One reason to keep things private is being able to work (and stumble along) with peace of mind it's unlikely others are doing the hard work of figuring out how to execute correctly. If others know you're working on it, they may take that as the validation they should be working on figuring it out (and possibly failing) too!