Lawn in the 'Hood

For four years now, I have been watching my neighbor slowly grow his lawn. I laughed when I first noticed he had planted a small clump of crab grass in the center of a square of dirt on one side of his walkway. But he watered it diligently through the summers, and even ran his electric mower over that little patch every week to keep it nice and trim. Now, four years later, it has spread almost completely to the edges. By the end of next summer, he will have a solid bit of lawn.

This summer, after killing off the weeds and carefully turning the soil, he planted a single clump of crab grass on the other side of his walkway. In another five years (assuming the other side doesn't die off before then) he should have a beautiful lawn spanning the front of his home.

I've reasoned that he chose this method for economic reasons, or perhaps convenience. He could have procured the clump from a friend, or maybe from his back yard. I've never planted a lawn myself -- maybe seeds are expensive? Or hard to come by?

What I really want to know is why he didn't plant both sides at the same time. Ten years seems like an awfully long time to wait for a lawn.