They usually just don't insist that strongly because they'd rather not get into a confrontation, as not moving might also anger people other than just the one you're holding for (as an argument I once saw happen on a bus went after a driver insisted, "the money isn't coming out of your paycheck so why do you care?").

Previously the consideration used to be nice because it was possible to end up in a situation where your metrocard was too low on balance, too far away from a machine to add more money to it, and you didn't have change. But nowadays most busses have tap-to-pay, so there shouldn't be many excuses.