'm rather curious about those teachers who dance socially with their students.
In any activity with teachers, or coaches, we should all know of the bad stories of that 'special' student targeted by the, usually**, male teacher.
You have such talent. Let me give you some extra lessons to bring that out. Quite a few instances on FB and in the media. So, yes, it happens!
**usually male, but not unknown for female. I've tried not to infer male or female in the following.
However, apart from that, there are numerous reasons:
Just friends/acquaintances - we (my wife and I

) sometimes each dance a tanda with a couple we know, who teach, and perform in BsAs. Occasionally take a lesson with them, but we also used to dance with them before that.
You've already proven yourself as an extremely good dancer. You're not really 'advanced' unless you can dance with a beginner or novice. It's social. It includes everyone. It can be fun. It's advertising yourself. It gets someone on the floor who may not invite/be invited otherwise. You do find some may have a special favourite (which may develop). You find some may restrict themselves to their specific group (several reasons); others don't.
Of course, if you're still climbing the slippery pole to the top (wherever that is for them), maybe you're more selective. You only want to show yourself at your best. I've mentioned 'performers', who don't wish to be injured, who may be seen to be selective, but a very different and valid reason.
I'm sure there are many more groupings which could be made, but so what? There's threads here on why you don't choose someone and why you do. All forms of human life can be found at some larger milongas, or if you travel.