Why do we call people who think philosophers?
Because they went to the trouble of telling us what they thought. No, that can not be right. That would make me a philosopher for telling you what I'm thinking now.
Why do we put them in categories?
(I agree there might be some professional philosophers .. like there might be some professional artists ..)
"For the usual 'reasons' we categorize things. Speech can be seen as a categorizing act too eh. But anyways, it's the next point of coming to know their ideas. Like athletes are in the business of moving their bodies. Start with Athlete then go to Baseball player versus Basketball player. We come to learn/distinguish the different ways these folks move their bodies and what are important motivations for moving their bodies like this or that. Things get really fun when we start shootin the shit about one player's style and how it interacts with the previous levels of categories. So it's handy to learn the language of the domain (then later you can bust it up and show the nonsense and such)."
My view is that philosophers don't categorize, they create new categories of thought. For example, take the idea of the idea. This was created by MrPlato. Before the the idea of mental images was not a regular part of human thinking. Philosophers help make thinking possible. Think of them as restructuring the operating system of the mind.
Specialization and categorization are what relegated philosophers and philosophy in general, to the realm of the university and academia.