Because, whereas in a compiled langauge each thread consists of little more than a set of cpu registers, a stack and an entry in the system scheduler tables; in Perl, you need those: plus a complete interpreter, plus per-interpreter copies of all non-entrant state.

And as Perl began life as a single-threaded app, its internal state and control structures were never designed from the outset to be reentrant, which means there is of necessity, large volumes of per-thread duplication.