Why won't they? I now have an account for myself solely, my spouse solely, me and my spouse jointly...

In fact you can open an account for each of your children (provided they are over 18) solely, then jointly you and each child, then jointly your spouse and each child...

Because they are all each considered 'new customer entities' hence qualify for new relationship sign up bonuses.

As as long as you are not one of the parties, you can even qualify for referral bonus - you refer your spouse, your children, etc. for their respective standalone relationships.