There are 3 credit bureaus: Equifax, Transunion and Experian.  The credit bureaus are businesses, and one line of business they are in, is in selling leads to creditors.

For instance, competing mortgage lenders can pay the bureaus to be informed when someone pre-qualifies for a mortgage loan.  The home buyer might then be contacted by (potentially) several mortgage companies.  That’s just one example – another example would be the various credit card offers you receive by mail.

You can opt out of receiving prescreened offers/solicitations for 5 years at this website:

Per what I’ve been told, it takes 5 days for the opt out to take effect.

Within the website, I recommend reading the so very helpful FAQ

You may also want to sign up for the Do Not Call registry:

I’ve signed up for both the optoutprescreen website and the do not call registry, and recommend both.

Here’s an article from the Federal Trade Commission (FTC), a government entity, on the topic

I was once told by a credit expert that opting out from prescreened offers can improve your credit score. But, this FTC link states that it does not:

Learn more about credit: 

Learn more about credit inquiries: