We are working on it! We currently have testbanks for Computing Technology For All and for Discrete Math, and hope to add more. We are also working on a testbank tool that knows your zyBook's configuration.
One reason we don't have testbanks for all our content is that interactive content is harder to create than traditional textbooks -- lots of details to get right, plenty of platform concerns, and more. Plus we are continually improving our content (no static "editions" model here) so keeping a testbank in synch is harder.
We also want to provide a more modern form of testbanks. zyBooks are configurable, so testbanks should ideally adjust to your class zyBook’s configuration.
We know that making exams takes time (we are teachers too), and testbanks help. We are as eager to work on testbanks as you likely are to have them, and work is underway.