That's because photography isn't a 'profession' in that sense:
Professional footballer
Professional musician
Professional sculptor
etc.
These are people who do things for a living which many people do for fun, which is I suppose where the requirement to add 'professional' comes in to differentiate them from amateurs
The difference with photographers is that they sometimes have an air of the other type of 'professional' but personally I think it's a red herring, and why this 'issue' comes up on forums on a regular basis.
Being 'professional' and being 'a professional' are 2 different things. And no matter how much we'd 'like to think' something is so, there's no line to measure against. Being able to earn money with a camera requires more business skill than photographic skill, and you don't necessarily have to be 'professional'.