Why do sites don't use coupon codes instead of discount when checking out never work?

Coupon codes might not work during checkout for several reasons, such as expiration dates, usage limits, or specific terms and conditions that restrict their application. Additionally, some sites might have technical issues or glitches in their code that prevent proper validation of the coupons. Lastly, certain discounts may be automatically applied at checkout, making the use of coupon codes unnecessary or incompatible.