I found this interesting article on square foot gardening. The basic idea is that you don't plant your vegetables in rows, but in 4' X 4' squares. The idea seems interesting. I'm trying a limited test this year. I didn't use the special soil they call for, just the dirt in my garden since I found out about it after I had already prepared my garden for the year.
One of the questions is "Why do you plant crops in rows?" Basically the answer was "Because that is the way we've always done it." That is why I planted in rows too. It doesn't make sense though for smaller gardens like most people plant today.
But how did it get started? I have a few ideas: 1) irrigation. It is easier to irrigate down straight rows, so if you are irrigating it make sense. 2) Plowing, straight lines would be more efficient.
Why plant more then thin? My guess is that if you are depending on the food to feed your family, you'd rather have too many plants than too few. Also, you don't know how good the seed is, so you'd rather overplant in case a lot of it doesn't come up.
fortunately for me, not sprouting doesn't seem to be a problem for me. My giant sunflowers seeded down last year and now I have 20 baby sunflowers growing. I'm going to have to start pulling them up soon. This is in addition to the lettuce and cillantro that seeded down.