You're right, for some things people will have already intuited it, and in a large enough group someone will have intuited pretty much everything. But many people will also have the wrong intuition.

The danger here is if we find data that goes against our common sense, we have to really consider that data, not reject it and rely on our common sense only. Sometimes it's right, sometimes it's wrong. Having a study prove something by no means makes it definitive, but it does provide better evidence and lets us better know if our intuition aligns with reality.