"First, the problem is inherently hard." If that was all it was, the estimates would still, on average, be correct. Your second point is really the key: others

"First, the problem is inherently hard." If that was all it was, the estimates would still, on average, be correct. Your second point is really the key: others pressure create an incorrect bias. In an ideal world, there would be more conservative estimates coupled with pleasant surprises when delivery is ahead of schedule.