crabs

[colk100]

why don't hermit crabs grow their own shells?

[Loosehead]

Because their species has evolved to use other shells. You might as well say why don't cuckoo's lay there eggs in their own nests.

[gen2]

In simple terms it is all a matter of risk and cost.  The crab does not have to use up energy and body resources making a shell of its own.  The risk is that it is vulnerable if it outgrows its current shell and cannot find a suitable larger one nearby.  Evolution has gone down this path where there are plentiful resources (ie discarded shells).