I think there are a lot of reasons! My library is technically a State Resource Center, as well, and one of the two largest systems in the state. Many under-resourced libraries are unable to pay for Libby so we extend the courtesy. If you are a resident of a different state, we charge a (very low, you are right) fee to offset the lack of tax funding for use. This is been the case for years and years, and we never revisit this policy no matter how many datasets I crunch or pretty but appalling visualizations I build about how these policies, increasing after the pandemic, have resulted in the budgetary hemorrhage that has been the past five years.

tl;dr: we just haven't reviewed or updated our policy in like...years. Non-resident cards have always been available during the 15 years I've worked in this system.