At some point in the past
you once paid off something exactly on time.
No good deed goes unpunished.
It got you on their mailing lists.
Just BTW; it does NOT mean you have a good credit rating.
For a variety of reasons, I deliberately trashed my own credit rating a few years ago. I know my own credit history, and no credit officer in anything approaching their right mind would give me a cent of credit, or even a scent of a credit line. (Rotten pun intended).
They send me the same 'pre-approved' offers, just as often. here's the key fact; the pre-approved means nothing. To get the card, you have to fill out all the same forms and go through the same approval process as if you had contacted them and requested an application rather than being 'prer-approved'. The credit limit they mention also means nothing; IF you apply, and IF you are approved, they will offer a much LOWER credit limit than that.
They make their biggest money from the merchants. They rake off from 0.5 of a percent up to 3.5% depending on the nature of the merchant's business and the volume they do with the particular card services provider.
They want you to buy lots and lots of stuff on the card, pay it off and buy more. Every time you turn your credit line over that way, they get their percentage.