The obsession with Trilogies

[levcore]

Hollywood loves its trilogies. Every time something gets rebooted or an existing property is relaunched or even a new IP it's always mentioned that they want to make a trilogy. Where does this obsession with trilogies come from? Sometimes i think studios are too bogged down with the long term that they don't concentrate enough on just making one good movie and then see about getting to make any more. The obession with trilogies has also led to retconning of the first films in order to justify two more. I wonder why studios think 3 is the magic number as more often than not quality tails off (Matrix Revolutions) and it can sometimes taint the original by being retconned as a trilogy. Thoughts?