Peering Creates Value (5)
Selling transit is necessary and desirable, as that increases the number and size of your customer base, the group of people who pay you money.
However, you cannot sell as fast as the Internet grows overall, so the portion of the Internet which consists of your customers will decline over time.
Thus selling transit is too slow a means of increasing value. It also constitutes a chicken-and-egg problem: if you depend upon sales for growth of value, and depend upon growth of value to fulfill new sales, you cannot gain momentum.