Seeds and "young" seedlings of course can be grown that close together, but then when they start growing larger they have to be moved to a location with more space. If all the seeds you plant were to germinate and grow, it might put you in a jam for space! If half of them don't germinate and then half of those that do die then you would have plenty of room. It is just that when they are very small they can be grown that close together, not when then get larger!
