To be fair, anonymous, small mammalian pokémon can withstand thousands of volts of electricity, tiny birds can generate hurricane force winds, and coming in physical contact with sentient blobs of magma or pokémon literally on fire only results in a slight chance of being burned. Pokémon are just more resilient than you’d imagine, for reasons that are simultaneously mysterious and probably eldritch in nature.
That having been said, though, crustle gets by with a combination of exceptional engineering (the weight of the shell is actually distributed across the entire bottom, including any part dragging on the ground, rather than focused only on crustle’s body) and a complete refusal to skip leg or arm day.
The Pokédex According to Bill 