Most engineers don't offer services to the public. Every public building, bridge and road you've ever used were all overseen and signed off by a PE. Same with the infrastructure for your local utility companies.
Most software engineers don't provide services directly to the public either, but licensure would a necessary first step to regulating the field.
Having PEs sign off and oversee projects is not the same as all "real" engineers being licensed. If you believe that "real" engineering is the model for software engineering, then only a small fraction of software engineers would have a PE.
That's not necessarily a bad thing. At the very least, I think it's reasonable to require that certain software engineering projects have some professional oversight, but that doesn't mean that the intern needs to be licensed, just that the important architectural bits of a project are approved by a competent professional.
This idea that all other engineers have a PE is rampant in the software field, but it's just not true.