It isn't, but then one isn't entitled to call themselves Engineer, just like a village "doctor" isn't one.
There is no proof that the person in question would fair well in all domains that get taught during 5 years of engineering school, and follow up experience on the field, without a proper assessment from the engineer board.
I guess you are happy to go to anyone that calls themselves doctor as well, or buy a house from someone that has learned to put bricks together during weekends.
The assessment proves that one actually has the skills that are expected from any Software Engineer to fulfill the society expectations, technical, moral and ethical responsibilities, from the profession.
There is no proof that the person in question would fair well in all domains that get taught during 5 years of engineering school, and follow up experience on the field, without a proper assessment from the engineer board.