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.
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.
Because "retina" in the context of displays is an Apple-trademarked marketing term, which is why Apple is the only one who can build a "retina laptop", irrespective of size or battery life.
More to the point, another manufacturing aping Apple's tradeoffs means the two end up competing on price, which neither wants to. So, naturally, other competitors who are interested in claiming some segment of the price-insensitive market by competing on something other than price make different trade-offs -- in this case, better DPI and shorter battery life. For lots of laptop usage patterns, this is a superior tradeoff; for others, its not.