[iamcalledrob]

The cost to taking an additional passenger is latency.

The bus has to stop, let the passenger on, and later let them off.

This affects everyone else on the bus and reduces the quality of service.

[pocksuppet]

Then you need more doors.

You can see that a long-distance train has doors only at the end of carriages, and stops for several minutes, but a subway has doors about every 10 seats, and stops for 20 seconds.