Experian's customers are not the people who received the letters. Their customers are those doing credit lookups for various reasons, and paying for it. Incentives matter, and Experian (et al) have an incentive to serve their customers, which is why they gather so much information and make it expensive, difficult, and inconvenient for their data subjects to withhold information.

Unfortunately this state of affairs is going to be difficult to fix with regulation (you can bet they'll lobby hard against that).