Most reward programs are product-level: a cashback rate on a credit card, a fee waiver on chequing, a rate discount on a mortgage. Each program runs independently. The member who holds five products gets the same cashback rate as the member who holds one.
The billing system sees individual accounts, so it cannot reward a member for holding five products instead of one, or for how they use them.
Building a relationship-based rewards program on top of a legacy core means custom queries, manual eligibility checks, and spreadsheets reconciled quarterly. It can be done, but not every cycle and not for every member.