In most (all?) US states, you need a license to be a _Professional_ Engineer. This is a legal distinction that carries weight _and_ legal liability. It doesn't mean that you cannot "engineer" without a license. There's legal distinctions on what work a licensed engineer can and cannot do, and what work must be done by or under the supervision of a licensed engineer. e.g. a junior's work has to be signed off by a supervising PE. To become a PE, a licensed Engineering Intern must have first passed a preliminary exam issued by the state and worked under a PE for a minimum amount of time with satisfactory performance (think apprenticeship).

It's the legal liability that SWEs don't want. e.g. You wouldn't be able to just slap "This is free software; see the source for copying conditions. There is NO warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE." on your software and walk away from it when lives are at stake. A so-called Profession Software Engineer would have to accept liability for their work product, and those of the juniors working under them.

I started down the road to becoming a Professional Engineer while I was in college as I studied Electrical Engineering and was more interested in hardware/power engineering. I took, and passed, the Engineering Intern Test in my state (which is the preliminary exam you take before you can start your journey to become a PE). Then life happened and job/career opportunities took me in a different direction/industry, and I've been doing software instead. So, I never became a licensed Professional Engineer. I'm still a licensed engineering intern, though. I'm still bound by the legal and ethical standards that it entails and could lose my license for violations.