That might very well be true but that is not who I am. I'll fly Delta and United, maybe Alaska, but not Spirit. I have my secret formula about how much I am willing to pay for certain things. In the past, there was some loyalty and there's still a desire to have loyalty if it is reciprocated by the airline.
You’ve described yourself as a price sensitive shopper who is willing to select between several airlines. You’re not the customer Delta “wants” to have a loyalty relationship with. They believe your purchase patterns demonstrate that your business is interchangeable with millions of other people who are also shopping for the lowest fare among multiple carriers.
Let’s also acknowledge that Delta is a company, and companies have no feelings. Consumers feel loyalty. Companies take advantage of our feelings and get us to engage in behaviors that go against our self interest (pay more, fly undesirable routes or times, etc). You’re saying that you’re not willing to pay “a little extra” for the sake of a “relationship” with Delta. That’s totally reasonable. But that decision also comes with the intentional cost of Delta saying they don’t want to spend anything extra for the sake of having a relationship with you.