At least in civil, the reason is because the professional engineer (PE) stamps all plans and assumes responsibility for said plans by doing so. Plans cannot be built without a stamp. This is the case because someone has to be found liable if a bridge should kill people, and it shouldn't be the technicians, designers or EITs under the PE, because they don't make nearly as much. With great pay comes great liability.

Not only a certification, but in Italy you need to be registered to a register/bar and pay a yearly fee in order to be an engineer.

It's like being a lawyer in some ways.