what's up with stores not sending their signup coupons?

Question: every time I do one of those "sign up for our email list, get 15% off your first order" coupons, I never seem to receive it (though admittedly, if they send it days later I've usually moved on and bought from a different website or decided against splurging on the purchase). I try using multiple emails from different domains and I don't get anything in my spam inboxes either. I've had this problem with many different popular websites, most recently with the TRTL travel pillow website - and I'm traveling next week, so would need to order soon if I order from them. Is this something companies do on purpose - not send the email coupon until days or weeks later, if ever? At which point I'm no longer interested and have bought something else? I'm having trouble seeing the reasoning in that?