A PPL is absolutely not a guarantee of competence. You might have a licence, but that's it.
Someone who has 45hrs under their belt is in no way shape or form competent enough to be entrusted with fare paying passengers, who demand a certain level of certainty that they won't end up smeared across the earth. I wouldn't say that most bare hours CPL's are upto snuff really, but at least they've had far more indepth training and been tested to a higher level.
The simple answer is that it is enshrined in law, but the law is there because the travelling public needs to be protected from cowboy operators as much as possible. PPL's aren't the cowboys, but they would be the ones taken advantage of if there wasn't a decent level of oversight of Public Transport. Whether the CAA provide that is an argument for another day!