From the way the question is stated you seem not to understand how important is the role of supervisor for a Ph'D.
With very few exceptions, it is the supervisor who chooses the project/problem for the student with the following goals in mind (among other things):
- try to make sure that the problem/project can be completed by a student.
- try to make sure that the timeline for the project can be completed within a reasonable period.
- make sure that the student learns the needed results/techniques/tools needed for the project
- provide guidance whenever when the project hits an impasse.
A Ph'D student most often cannot chose his project for himself/herself/itself. If he cannot do that, how can he chose one for another student?
Also, how can a student evaluate how hard and how long will take for a project, a project which often needs tools from areas he is unfamiliar with? This often is a skill gained after years and years of research, and even then we are wrong from time to time.
Besides working on a project, Ph'D is also a venue for gaining knowledge. This is usually of two types: basic knowledge in the mainstream of that (sub)field, and specialized knowledge for that project. Ph'D students usually know where/how to find the first type, but they most likely need help with the second. How can a Ph'D student recommend another where to gain knowledge he is unfamiliar with? Most likely, the "guiding student" will have no idea what "his student" needs to learn.
Last but not least, when a project hits a problem, it is often a subtle problem: either something most PhD students cannot solve or often one where the PhD student has to learn something he is not familiar with yet... How can then he help another student in this situation?
If supervising a Ph'D would be a punching card job, where you just go to work for 8 hours, and do repetitive tasks than your suggestion would be fine. But it is not.
Ask yourself the following question: how confident would you be in your surgeon, if he is a resident which is learning on the job from another resident?