After the first, short paragraph addressing the strict issue with "retina", the longer, second paragraph of the grandparent post directly answered why that would be the case: competing on different feature trade-offs rather than making the same trade-offs and competing on price allows greater profits because it avoids entering a market with direct substitutes, even if doing so in a way constrained by the trade-offs earlier entrants have made results in entering what is effectively a smaller market without direct substitutes instead of a larger market with direct substitutes.
Why is Apple the only one that can build a 15" retina laptop with 9-10 hours of battery life?
The answer "Dell makes a laptop with a better-than-retina screen with terrible battery life" is not in fact responsive.
An easy answer to that question would be to submit for consideration some non-Apple laptop that accomplishes that. I'm sure it must exist, and that would be a good thing to know about.
That there is some random Dell laptop with better resolution than a retina display and worse battery life is simply not responsive to the question.