Why are AMD and Intel so closely match in terms of processor power?

If you separated two groups and incentivized them to develop the best processors and came back in 20 years, I wouldn't expect both groups to have done approximately comparably. Particularly so if the one that is doing better is given more access to resources. I can think of a number of potential explanations none of which are entirely satisfactory to me, though. Some possibilites:

• there is more cross-talk between the companies than I would guess (through hiring former employees, reading patents, reverse engineering, etc.)
• Outside factors matter a lot: eg the fab industry actually determines a lot of what they can do
• Companies don't work as hard as they can when they know that they're slightly beating their competitors (and the converse)
• Selection bias: I'm not comparing Intel to Qualcomm or to any competitors that went out of business and companies that do worse in performance would naturally transition to other niches like low-power. Nor am I considering markets where there was a clear dominator until their patents expired.
• Basic research drives some improvements and is mostly accessible to both

Though none of these are particularly compelling individually, taken together they seem pretty plausible. Am I missing anything? I know basically nothing about this industry so I wouldn't be surprised if there was a really good reason for this.