Anyone who has worked down a list of discount codes, watching each one fail, has met a well-known feature of coupon sites. The codes are stale because the site is not paid for them working.
The payment comes from the click, not the code
Coupon aggregators are affiliates. They earn a commission when a shopper clicks through to a retailer and completes a purchase within the tracking window.
Revealing a code opens the retailer's site in a new window, which sets the tracking cookie whether or not the code is valid.
A shopper who tries five dead codes and then buys at full price still produces a commission, so the incentive to remove expired codes is weak.
Long lists outperform accurate ones
A page showing many codes ranks better in search results and holds attention longer than a page honestly stating that no offer is available.
Each attempt is another click and another chance for the tracking to attach, which makes volume more valuable than accuracy.
Some sites soften this with success rates or user voting, but those signals are self-reported and easily distorted by the same incentive.
Verification is harder than it appears
Testing a code properly requires building a real basket that meets the conditions, since many offers apply only to specific categories, minimum values or account types.
Targeted codes cannot be verified at all from outside, because they only work for the accounts they were issued to.
Doing this continuously across thousands of retailers is expensive, and the revenue does not depend on it being done.
Retailers have mixed feelings about the model
Commission paid on a shopper who was already at checkout is money spent on a completed sale, which is the least useful marketing a retailer can buy.
Some respond by excluding coupon sites from their affiliate programmes, by removing the discount code field from checkout, or by applying reduced commission rates to that channel.
Others accept it, on the view that the sites capture shoppers who would otherwise leave to search and not return.
What actually improves the odds
Codes issued directly by a retailer to an account holder work far more reliably than anything scraped from a public list, because they were created for that shopper.
Browser extensions test codes automatically, which saves time but sets the same affiliate tracking, so the commission is paid regardless of the result.
The realistic expectation is that public codes are a low-yield search, and that the time spent is often worth more than the discount found.