I do not know European regulations very well, but in America far too many jobs are bound to regulation and certification requirements than is reasonable
It isn't difficult. You're "professional" if you are charging people for transportation services. And if you are doing so, your standard car insurance isn't going to cover you in the case of an accident.
This isn't some case of over-reaching government; if you kill or maim someone while driving from point A to point B, the likelihood of which increases as a professional driver spending more time on the road, you need to ensure that you can cover associated costs of said disaster.
People fail to see that at least part of the reason Uber is able to be cheaper than a taxi service (much like AirBnB is cheaper than hotels) is that they are skirting a requirement that taxis have: sufficient insurance.