Many times software engineers fall under the 'industrial exemption' for most states' professional practices acts. That's how many folks without a PE are able to be engineers - as long as they're not selling engineering services to the general public, but only to their employer.

Things get trickier with software as well - with many engineering disciplines, the public can be placed in harm's way by the malpractice of engineering - that's why many civil firms require PE's. In other areas, it's less desirable - for defense contractors, many purposely avoid PE's due to our ethical obligations around harming the public.

Surrounding software, I think a major part of it is just that the laws haven't caught up with the times. NCEES now has a Software PE available but no clue about the contents of the exam. It seems like a tricky one to test for.