There are several reasons why someone may not publish such works.
For one thing is that if you are the student and your teacher gave you the arrangement then you, technically, have no right to publish it. It is not your arrangement and publishing without the consent of the arranger (or their estate if they are deceased) would be against the law.
Although to defend a copyright you basically have to have it registered, technically as soon as you write something it is considered copyrighted. Only the holder of the copyright (the author unless legally transferred to someone else) has the authority to publish.
Another reason is that publishing isn’t necessarily easy. If you self publish you are basically setting up a second business. If you have someone else publish you have to find a publisher who is willing to publish first, and then deal with all of the legal and financial issues surrounding publishing a piece.
Many teachers who do their own arrangements have no idea where to even begin, much less a student who just had an arrangement handed to them by their teacher.
My teacher was as much as composer as a music teacher and since she wrote so much music she started her own publishing company to handle her compositions and arrangements, as well as those of some of her students. Most of her compositions are available to anyone who wants to buy them.
However, she was for ever refining her arrangements and I have copies of several that have her hand written changes in them. Some have some real significant changes to them. Some have been incorporated in to later editions of the published piece and some where not for what ever reason.
Am I going to publish the versions I have? Of course not. If she wants them published she will publish them. When she dies her estate will make those decisions. I have no right to do it. The versions I have she did for me. I am not covetous of them and have no problem with someone else playing them, but she didn’t decide to make them all available through publication so who am I to make that decision for her? I simply don’t have the legal right to do it if for no other reason.