You can make the same argument (in terms of space) about a train track. The real advantage of trains (light or heavy) is permanence. It's easy for the next government to remove the bus lanes because "OMG too much traffic, one more lane will fix it." It's much more difficult to rip out a rail line and convert it to a road.

A rail track for the same route sits empty just as much.

> you can add additional lanes almost indefinitely, and it doesn't really help congestion that much.

I don't think adding bus-only lanes would have that effect. Adding lanes for private vehicles reduces congestion, which encourages people to move to places along the route until the congestion reaches the previous barely-tolerable level (as I understand it).

Busses aren't much more efficient when riding down a lane than lower occupancy vehicles, but streets aren't bottlenecked by their roads, they're bottlenecked by their intersections. The key advantage of a bus is at the intersection. A bus holds the intersection for far less time than the equivalent passenger capacity of cars.

The problem bus lanes try to solve is dominantly that without them the traffic advantage of people riding busses mostly goes to people not riding busses, and this makes for a pretty terrible incentive structure. Busses are intrinsically disadvantaged against cars (schedules, uncertainty, routes), so if you don't help them, then people will prefer to drive. Bus lanes internalize the externality.

While that bus lane may look empty most of the time it likely carries far more people per hour than the congested car lane next to it.

> While the Lincoln Tunnel’s car lane can only move 3,000 people per hour in each car lane, its bus lane moves 30,000 people per hour.

https://transalt.org/blog/bus-commutes-are-significantly-lon...

And your bus only lane has a lot more options. If there is a major disaster you can divert other traffic (not necessarily all traffic though that is an option) into it which might be a useful compromise at time. If you need to repair your bus only lane you just divert the bus to regular traffic. For that matter most places there isn't any traffic and so a bus in mixed traffic has no downsides thus not costing you that whole lane (or track), just build the bus-only lane where it is needed.

Trains are a good thing when they do something a road cannot. However the common bus can be just as good for much less. If you have the money and want good service and ride quality the bus can do it too, and typically for much less cost than a train.

Trains are good where they don't mix with traffic (meaning elevated or underground) because they can then be automated (and also faster). Alternatively a train can hold more people, so if you are in the rare situation where a 100 passenger bus every 5 minutes can't handle the passengers a train is good. Most of the time though you are not in either situation and so a bus can do everything a train can.

Other advantages: people who don't drive, which includes children can get about. Lots of public transport can compensate alot for un walkability of suburbia.