The problem is more people than not are at least content with the existence of the prequels. A reboot would be terribly difficult to market, and may make a great deal of people upset who thought at least episode III was fine. It’s far less confusing and easier to pop out a spin off every other year. Disney does what they think will make them the most money in the most feasible manner, and the OT and its imagery is ten times the money-maker the prequel-era will ever be.
What I could see them doing are a series of movies based on Obi-Wan’s time in exile in between III and IV. I mean, you’ve got twenty years of stories to tell there, and if you really wanted, you could sprinkle in flashbacks to the prequel era with Haden and Ewan, and lift their characters up higher to what they were suppose to be (for example, show a scene where Anakin looks at his lightsaber admirably and boasts that one day his children will take it up and carry on the Skywalker legacy, or something like that). That would at least aid the cause to “fix” the damage caused by the PT.
I completely agree with you, but I’m sure they will reboot Star Wars eventually: considering today’s movie industry’s trend, I’d say 30-50 years from now (when the “saga” may be considered “complete” to its original fans or “too old” for the audience) Disney will be very eager to completely reboot the franchise to milk it some more. I understand that Disney got this strange illness that drains all creativity, just like all the other major studios. Fortunately, they won’t risk to be infected by Competition so long as only these (very few) studios together have enough capital to control the industry 😄