Buses are much slower to accelerate, and unlike most cars, you can't see what is ahead of the car in front of you.
When you get a chance, follow behind one during a non-congestion period and count the cars that pass you in the other lane (meaning, your lane is slower) and estimate how many additional cars it would take to have the same speed of traffic (how many cars would need to be in your lane, instead of 1 bus, for the lane to be as slow?).
That behavior inconsequential during non-congestion hours as the traffic can flow around the bus or else there'd be congestion, and helpful when traffic is near congestion. Traffic models show that it's the sudden acceleration and esp deceleration that "propagate" and cause congestion, accelerating slower and staying (at a safe distance so you don't have to hit the brakes) behind the bus are helpful.
Imo everyone driving unpredictably to "get ahead" and save (literally, do the math) 1 minute on a 30 minute commute is what causes a lot of congestion.
Imo even if a bus took up 10 car spaces, the more predicable driving and slower accel/decel might still be a net positive for traffic flow.
Traffic congestion increases nonlinearly[1]. i.e. the 10th car on a road has a smaller effect than the 100th car on the road. Accordingly, the cost of increasing a bus' size and decreasing its speed pales in comparison to the value derived by removing 5-20 cars from the road.
But there have to be 20 people inside the bus (approximately 50% utilization) for that effect to be seen. Do you see that many on the bus at non peak times?
Realistically a bus is 2-3 times the size of a car. I think you need like 5 people to justify it. And 6 before you're actively addressing traffic. Most bus capacity is around 10. Which is 4 cars saved.
Also, do you care about the state of traffic at off-peak hours? who cares if buses don't run at peak capacity when there's no traffic?
When you get a chance, follow behind one during a non-congestion period and count the cars that pass you in the other lane (meaning, your lane is slower) and estimate how many additional cars it would take to have the same speed of traffic (how many cars would need to be in your lane, instead of 1 bus, for the lane to be as slow?).