Point of a single agency is not just to collect taxes. It is to make sure there are better outcomes, so that when you do need more taxes to fund public transit, you don’t get the pushback from the public.
The goal (by law) should be twofold:
1) increase the number of people with a 90th percentile commute under 15, 30 and 60 minutes. (i.e., improve quality of life and encourage economic growth)
2) sharply reduce the average CO2-equivalent emissions of each commuter.
The current statewide policy (by law) is to reduce commute miles.
In practice, this means intentionally sabotaging commute corridors, which slows economic growth (fewer available workers and fewer reachable jobs) reduces quality of life and increases CO2 emissions (due to repeated idling and hard acceleration).