I think for some it taps into the same reward neurons as winning £10 on a lottery after paying £1 in week-in-week-out for years. It feels like a win, and that for many overrides any desire to properly analyse the matter (did I actually save, with the coupon, or save 5% on something that has been marked up 20%? (or buy something I didn't really want at all?!)). Same with BlackFriday, many of Amazon's “prime day” offers, and so forth.

[Also not American, I'm a UKian/ex-EUian, it is not uncommon to see the same here, just not in the big way some Americans tend to go with almost anything]

People will buy a thing that is 90% off reduced from $99 in preference to buying the identical thing for $5 with no discount.