I agree that it's definitely a thing that smaller businesses are very involved in.
I purchase from small businesses a lot and it's very common to find discount codes off my next order inserted into the package when it arrives.
Not to say that big businesses don't do it also because I've had the same off major retailers, but it's less frequent in my opinion. I would say it's because big businesses have the big budget for advertising and so can lean on this as a marketing method (for gaining new business as well as retaining existing) and don't have to work so hard at customer retention through loyalty discount.
ASOS have one of the best loyalty schemes around, it has to be said, despite being a large company. You can pay a small fee for an entire year of free next day delivery and they have a loyalty point system too.
Amanda