I think it is a deeper, subconscious psychological trick. "It's a special offer and it is limited time, I should get it while I can." Like, appeal to opportunists, or something? Of course, you're completely right and we rational people just override this and do not fall for such cheap tricks.
It's a standard marketing trick - because it works.
It's why shady online marketing sites always have a "Special discount for today only!" tag. And - protip - why some automatically offer a 10-20% discount if you try to bail on the transaction.
Giving people just one year of simple economical education about such stuff would probably have a huge financial lifetime gain for large amounts of people. Sad.
