Back to the original question...I am only a rec coach and have very few insteances of working with adults except in running cariod tennis or feeding balls. But I have worked with, and have a lot of mentors that I work with, who do coach adults on the regular in both private and groups classes, and in private, resort, and public settings. My observation is less about the coaches motivation to teach a particular student, but the students motivation and ability to learn.
Let's not kid ourselves that there are coaches that just are awful, no matter who they are with. I have seen plenty of those Chubbs coaches and can remove them immediately. They do nothing for any student.
But given a coach is interested in actually coaching, I see a large group of them working with kids. More that there are tons more kids that parents are pinning hopes on, or need activities to occupy them with, than adults looking to improve in something they do for fun. So simple opporuntities skew things there.
For those that do work with adults, that is where it comes down to the motivation and ability of the student. As was mentioned inthe thread, some students won't have the athletic ability to perform much better than they aleady do, or ability to improve technique. A good coach will work on tactics and other attirubutes to improve their game. Some students have been doing the wrong things for so long you really can't modify the behavior without extenisve work and many students will not understand that and blame the coach for not 'fixing' the issues in a lesson. And again as was mentioned, some seem to want to tell coaches what they need to improve over actually understanding what needs to improve, so the student won't progress like they can. Maybe there are coaches that just outright dislike working with adults because of the negative profiles of some adults like that, but I haven't found tht many myself. I have found those bad coaches, a large majority of good coaches, and then some elite coaches that I tend to try and hang around as much as possible to learn from.