I suspect that if there is a niche, something will evolve to fill it. The question is why is extreme toughness at a cellular level the right way for a larger animal? The square-cube law means that the larger an animal is, the more easily it can maintain its body above freezing point all winter.
If we look at the Arctic, there is a frog which survives freezing solid every winter. It hasn't evolved extreme heat or vacuum tolerance because it doesn't need them. It is also an outlier. The approach used by most Arctic critters is good thermal insulation. Many hibernate to a greater or lesser extent. They reduce their metabolic rate and body temperature so they can survive winter on little or no food without freezing.
This approach works (for birds!) even in the harsher Antarctic. The Emperor penguin is perhaps the ultimate example. It goes somewhere so cold abd barren that there are no predators in order to breed safely, surviving purely on insulation and body fat.
If Earth's seasonal variations were even greater or longer I suspect there would be larger animals managing to survive freezing solid for lack of any alternative evolutionary adaptation. The frog is cold-blooded so this is the only avenue open to it to survive in the Arctic niche. For warm blooded creatures, it is easier to evolve insulation, fat reserves, and hibernation.
Earth does not have vacuum or high radiation areas so any ability to survive these is an evolutionary accident, rather than something selected for. There are geese that fly at altitudes where airliners cruise and humans would pass out, but there is no evolutionary reason for them to go any higher.
