The cold hard reality as simple as this:
They want to attract new customers. Maybe members who haven't bought anything yet, maybe also hoping word will spread around the Poserverse and new people will join just to take advantage of the offer.
Current members who have bought stuff already are just that. I'm sure they're "appreciated", but the likelihood that they will buy more without any sort of inventive to do so is pretty good.
If I were running this business, the last thing I'd want to do is have people get used to the idea that they get tremendous sale offers frequently, or anytime somehow not doing so seems "unfair". You don't want people getting too used to that sort of thing, because then they expect it more and more.