I think the reason is that the incentive of the carpool lane is to get fewer cars off the road, not to literally reward the act of carrying more than one person

I think the reason is that the incentive of the carpool lane is to get fewer cars off the road, not to literally reward the act of carrying more than one person in a vehicle. To that end, taxis can use carpool because someone using a taxi is someone not driving a personal vehicle (and parking it at their destination, and driving back home later). If taxis are good enough, people won't buy cars, and fewer owned cars generally means fewer cars on the road.