Originally Posted by
tpi2007
Designing a CPU, especially one from scratch, as was the case with Zen, takes many years. What happened is that they hired Jim Keller in August of 2012 to work on Zen once they realized that Bulldozer was a dead end. Meanwhile they kept iterating Bulldozer the best that they could to follow the original plan (and to have something minimally worth buying), while Keller and AMD's other engineers built something from scratch.
It takes time. Designing, validating, platform, etc. And then when the design is finished you need to see if the manufacturing process you need is available or you're going to need readjustments. And then there's the back and forth with engineering samples and the making of the physical dies which takes months until they have a design with a 'viable' number of bugs left (every chip has bugs, called "Errata", the serious ones have to be fixed before launch, the others can usually be worked around in firmware and / or software).
Intel, having no competition, focused on improving power efficiency in order to be viable in thin laptops and tablets and to fend off ARM, and also on their iGPUs, which really needed the effort.