I have been asked to step out and formally identify my belongings before my car was searched at a land border so it does happen too.
Anecdotally, in trains, I have experienced very different treatments depending (apparently) on the class of travel. Profiling (based on appearance, type of car, etc.) does happen too in many places. So the treatment you get might depend on some (subjective or systematic) evaluation of the risk you present and/or the level of arbitrariness you can be subjected to without making trouble. And someone travelling by bus is unlikely to have powerful connections…
For example, in the cheapest class in a train, I once saw customs and police (three different agencies and six or seven agents in total) get on board early in the morning and spend half-an-hour asking loudly which bag belongs to whom, checking passports, phoning in to check mobile phone serial numbers against some database (yes, really). Travelling in a sleeper on another day, at the exact same border crossing, I handed my passport to the attendant in the evening and did not even have to wake up or open the door of my sleeper for the border check.
With collective means of transportation, another factor is that once you are out of the bus and waiting in line with your stuff, it's easy to pull you aside with minimal fuss and delay. Note that contraband is not the only thing police would be looking for, they also want to be able to filter people/prevent them from crossing the border illegally.
It would be more difficult if they would ask specific people to step out of the bus and identify their bags when it's already clear they have been been flagged for secondary inspection or worse. As @pnuts commented, people could also be tempted to “forget” their bag if something illegal has been found in it, which is not unsurmountable but would also cause delays.