So many discounts to sign up, but zero incentive to stay

There's so many "free boxes" promotions out there but only for new customers. I haven't seen a single promotion for existing customers. They incentivize people for signing up but shouldn't they be incentivizing us to stay and order more boxes beyond the promotional period? Why haven't they implemented a rewards program for customers? Something like 10% off every 5th box or something would be nice for loyal customers and keep us coming back every week.

If there is such a thing, please let me know how to enroll! I've done 60+ boxes now.