There is an extreme difference in the number of hours and required training between your SPL/PPL/CPL/ATPL. Eg: For fixed wing, you can get your PPL within 35 flight hours at a Part 141 School. Your ATP comes at a minimum of 1500 hours, and you are required to know much more about emergency recovery. The regs in place regarding minimum requirements for carrying passengers and performing commercial ops are there for very good reason. Even when having your PPL(or other license for that fact), there are currency requirements on when you are allowed to carry passengers on Part 91 non-commercial flights.
In the aviation world, even having your commercial is considered a "license to learn". Having your PPL simply implies that you can fly an aircraft in good weather conditions, should have the better judgement not to fly beyond your weather minimums, and have the ability not to kill anyone outside of your aircraft should conditions deteriorate while in the air. Even with my fixed wing PPL, I am extremely cautious with passengers knowing that I don't have nearly as much knowledge and experience as someone with their CPL/ATPL.
Source: PPL w/ instrument Fixed Wing & CPL-H