Because you can allocate the activation record on the heap, provide a way to reify continuations, and presto! now you have call/cc (and trivially implemented, no less)!

Price paid: a) you need a GC (ok, whatever, sure, you have one), b) well, the performance hit of having to GC activation records (you don't care; others do), c) whoops, thread-safety got much harder and maybe you can't even have threads (imagine two threads executing in the same activation record at the same time!).

That's a very very steep price.