I guess they've painted themselves into a bit of a corner with the way they do their pricing, and product announcements with a lot of hype.
If they were to do the kind of update you suggest, there would be a lot of bad press stating the obvious ("It's just the same as before, but faster and more RAM. No new alloys or anything!"), and it would drive down prices for inventory of the old model.
Apple has stated they think the iPad Pro is the next computer for everyone. I would bet Apple is quite far along making tools for the iPad Pro that let you create apps for the iPad Pro and iPhone. They released a first foray into this kind of tool last year with their Swift Playgrounds, a language they designed to replace old things, using all-new APIs like Metal. The computers we like all stem from what we call Apple's history and legacy!
This makes a lot of sense when you wonder why they aren't putting a touch screen on the Macs. Microsoft took a desktop OS and bolted a touch screen onto it. Apple will take a tablet OS and bolt a keyboard onto it.
[edit] Makes even more sense when you consider who has what apps. Microsoft had a huge lead in Windows apps, so of course you want to bolt your touchscreen onto that. Apple has the huge lead in iOS apps, so better to bolt a keyboard onto that than a touch screen onto the Mac.
Indeed, there is no announcement either. The price/spec just changes on the Apple Store. Most of the time, the reaction is positive - people just like to see that Apple care.
Keeping the same price for a machine over 3 years and not even going through the motion of offering spec bump is no very considerate. That really looks like Apple keep the line just for that sucker that is born every minute.
If they were to do the kind of update you suggest, there would be a lot of bad press stating the obvious ("It's just the same as before, but faster and more RAM. No new alloys or anything!"), and it would drive down prices for inventory of the old model.