About as lame as if you could go to a T6 Forest Planet and find Solarium, Aegisalt, etc. More Harsh Environments = Harder Planets = Higher Tier. Sounds reasonable that volcanic planets are high-tier. The environment is very harsh, and only the strongest monsters could adapt to live there. If they can withstand the environment and thrive there, then they must be strong indeed, and as such, are difficult to kill. If they were easy to kill, then they wouldn't be able to survive the harsh environment. Makes sense in a round-about way.