The third one is the killer factor: it makes sense to hire a train crew for hundreds of passengers, and the labor cost will be a comparatively small part of the trip's total costs.
But for taxi services, the average number of passengers is only ~1.4, and it requires a dedicated driver who will idle about ~50% of the time even in relatively busy metro regions.
I.e. the utilization of taxi drivers is very poor: one taxi driver is transporting only one passengers on average if we consider idle time.
So even if we assumed a competitive environment for trains (which it isn't in most markets), eliminating the driver for passenger cars via FSD is a more than 100x times higher cost advantage than for trains. (!)