Well, the problem is that licensing often morphs into cartel building over time. The ABA and state bar associations are notorious for this, but they're hardly the only ones. Read about how various bar associations have pursued "cease and desist" injunctions against "We The People" (a company that fills out legal forms for a fraction of what a lawyer would charge) for a sense of how far this can go.

If engineers want to include a software component in their licensing of, say, civil engineers, that's fine by me. But it needs to be clear that they are licensing civil engineers who write software as part of their civil engineering work, not software engineers per se. Once they get their hands on software engineering, and discover how lucrative the newly formed cartel (er, profession) can be, you'd be surprised with how broad the scope of work that needs to be licensed can become.