I don't understand why any "why" question in evolutionary biology is ever satisfied with a "survival of the fittest" truism. Any evolutionary explanation you give me is also falsified by the existence of another species with a different/opposite trait. Whatever explanation you give for a bee's barbed stinger is falsified by the wasp's non-barbed one. Also doesn't answer other questions, such as why didn't bees evolve…
In evolutionary biology, that definitionally is the ultimate answer. One species survived, another didn’t. Sometimes that’s because the adaptation helped them outcompete, sometimes it’s because they were already competitive and this preexisting disadvantage from an earlier round didn’t hurt enough to matter. We can try to find intuitive explanations past that which feel satisfying but it’s always going to be a rough approximation.
Let’s use chess as an analogy. Allow an engine to analyze a position and tell us the best path forward. But why did it choose that line? We can (and do) come up with explanations that help us fit a move into our understanding of the game: moving this pawn allows that knight to occupy a better spot where it can exert its influence on the rest of the board, or whatever. But that’s merely a convenient simplification for our gut understanding. It’s not really the actual answer. The ultimate “why” is “because it produces the best possible eventual outcome no matter the response”.