Even in the suburbs it is not hard to get a bus that is half full during off peak if the service is good. Most service in the US is not good so perhaps you are only looking at one of the many bad examples and incorrectly thinking it must be that way - it doesn't.

> . If we could modulate form to meet demand precisely, like Uber but for all transit, it would be far quicker and more efficient.

Efficiency is not the only or even the primary goal. The primary goals are to "Get people from where they are, to where they want to be, when they want to go, in a reasonable amount of time, for a reasonable price.

Your modulate demand fails at that. A fixed route bus is predictable. I know in advance when it will be there to pick me up and where/when it will drop me off. That means I can plan my day. With Uber or any other demand/response system you don't get that and have to hope every time that it will get you there in time.

Quicker is much harder to quantify. A bus can connect to a faster rail (which potentially can go 300 km/h - only a race car can do that). A bus can be given signal priority while the uber needs to stop and wait more. However a bus will stop to pick up others on the way. (but if your "modulate to meet demand" does that it is worse than either a bus or uber because you can no longer predict when you will get there). A bus network done right is also likely to drive changes in your city such that most of the places you go are where the bus is going and that makes it just as fast.