They have no idea whether you are something valuable or not. They are gambling, and it's their job to find undervalued companies. To them, there's zero benefit to ever closing the door. If they close the door, you won't come back, and someone else makes all the money. They will keep it open forever, in case your little company turns out to be the next Facebook or Google. They are not actively seeking to waste your time, and they are not wasting their own time. By not saying yes, and leading you on, they are telling you they don't see you as a gold mine yet.