One reason is because you would gain cents of fees but only in exchange for adding many many kilobytes of unsolicited commercial adverting to the chain. This ma

One reason is because you would gain cents of fees but only in exchange for adding many many kilobytes of unsolicited commercial adverting to the chain. This material must then be processed by all nodes and maintained for all time, and in doing so burning the tolerance of others to participate in the system. It's like asking why you don't litter when doing so would save you the few moments it takes to put something in the trash. The health of the system is more important to many than extracting a few cents more in fees.