This is because AFFIRM is what's called a "Consumer Finance Account". They are considered "bad" by credit agencies, because they are often used to provide financing to people with bad credit.
Unfortunately... Online retailers offer this type of financing without making you aware of that, so you might be tempted to take advantage of their credit offer. What you are actually doing is taking out a "bad credit loan".
That being said... With AFFIRM, they usually offer you a few different "plans". If one of them says 0% interest, then THAT plan will not even report on your credit report (unless you don't pay it). Any of their plans that DO charge interest will report as a "Consumer Finance Account" on your credit report. (which is a negative thing).
People with decent/good credit usually qualify for their 0% interest loans.
Similar situation with Credova, and other such credit offers. (I don't know if any of the other companies have plans that DON'T get reported, like the one AFFIRM plan).