That's a feature not a bug from an investor point of view. As a traveller among the big 3 American miles are clearly the most valuable, then United, then Delta. Since they all likely can charge about the same to their credit card partners, giving the consumer less value per mile is like having higher prices.
Delta has convinced customers in various ways (fortress hubs, lounge, service investments) that taking less value than their competitors is still worth it. Basically they're the price leader.