Because it doesn’t solve the problem, it merely works around it. Solving the problem would mean coming up with a battery chemistry that doesn’t suffer in the cold. Instead the answer is “just don’t have it get cold”.

It’s not to say a hack/workaround isn’t useful, and I would say that it’s perfectly acceptable to simply use a battery heater in the winter. But calling it “solved” confuses solutions and workarounds, and that’s an intellectually dishonest thing to do.