It's not about titles. It's about what you're allowed to do/practice. I can certainly call myself doctor without an MD or other medical degree for which the title is customarily used. I might even do so if I had a PhD. I suppose I could do so in any case although people would think it strange if I didn't actually have a degree the same way they'd think it quite odd if I insisted on being called Your Highness.

However, if I were to start practicing medicine in anything other than a first aid context or if I were to mislead people into thinking I was a certified medical professional, that's not allowed legally.

By contrast, while PEs are required especially for signing off on certain documents in civil engineering, there is not in general any requirement to have one to practice engineering in the general case. In fact, there's no requirement for an undergraduate degree.