Except you’re forgetting public unions, which lobby extensively and would benefit if tax filing were simplified (allowing taxes to be raised more easily). If spending the money would get the result, they’d do it.
It’s also possible that unions have other areas that are more important to them as a group (worker rights, fighting right-to-work, healthcare, etc). In fact, I’m not sure any group has as much interest in this as tax preparers, hence their annual lobbying.
