That's a good point. I'm sure the bus operators programmed the bus' route to make it at least as good as regular trains with regards to the basic performance metrics such as average operational speed.
> And if there are no structural issues, why do trains have so few doors nowadays?
In short, design options. If a railway operator felt a demand for passenger wagons with more doors and a reduced number of seats, it would be trivial to design one. There are downsides to this (less seats, more moving parts, more maintenance, potentially heavier wagon, harder ticket control, etc...) but nothing stops anyone from doing a passenger car with doors everywhere.
In fact, this was already done in the past.
In practice there were half a dozen non-structural reasons to move to less doors including automatic doors being much safer, disabled accessibility and simply that those trains were cold.
They also may be enough to annoy people so much that they will avoid this like the plague. Extreme example: if this loads and unloads like an airplane and makes a stop every few miles. Yes, maybe only 5% of passengers would get out and in at every stop, but that still would be a one minute stop every two minutes or so.
And if there are no structural issues, why do trains have so few doors nowadays? I would think stops could be a lot shorter at peak hours of trains had more doors.