A full bus is indeed far more efficient than a car. But buses run pretty empty most of the time. As https://reason.org/commentary/does-bus-transit-reduce-greenh... estimates, in 2009 buses emitted nearly double the CO2/passenger mile as cars! Prioritizing buses, meaning adding more, makes that figure even worse. And therefore as simple as the bus logic looks, it doesn't actually work out to better in the real world, in most American cities.
There are ways to fix that particular problem. But the place to really start is in housing permitting, NOT the streets. When you get sufficient density of housing in one place and work in another, then mass transit becomes efficient. But people involved in traffic planning seldom have the authority to overcome NIMBY obstacles to building denser housing. And so they have to operate under constraints that make additional mass transit a generally worse option.