I think it has more to do with the developers want people doing other things than just Coil all day, every day. I guarantee you there would be a subset of players that given complete freedom to run whatever they want, whenever they want, they'll have completed all the current content and geared every class way before the next content cycle was due to come out. I think SE would much rather people have people logging in once a week (and still paying their subscription) than whoring out the content within a couple of months and unsubscribing until the next bump in progressive content comes out.

It's simply a business decision.