If you're talking about places like Dell and HP, it's because they can engineer to a specification and place cooling exactly in the right place to reach a temp that they have already decided was acceptable. They can even setup their motherboards to have components in a place for optimal cooling. so they don't over-engineer a cooling solution. They also have the capability to engineer their cases to be sound absorbent and their fans always have PWM and temp limits and steps have been engineered and probably hard coded into the bios. they also don't overclock and tax the cooling system (or the PSU, whose fan and temp sensors are also related to the cooling system). The "OEMs" that overclock are usually very conservative and they use off the shelf parts so they are not as quiet as something from Dell or HP.