If so, why would PhD students need supervisors?
is a key to the situation.
From what I've seen, there are substantial differences on how this question would be answered in different (academic) cultures.
E.g. I was told/reminded by my professor early on during my PhD research* that I was a fully qualified chemist - with all professional rights and duties that chemists have by my Diplom (= MSc; here BSc and MSc are usually prerequisites for a PhD). The formal description of Diplom thesis is a 6 months research project that the candidate performs on their own (after "guided" research in the form of obligatory research internships).
Obviously, with a fresh Diplom, one isn't an experienced scientist. But I was largely free to decide my approaches and the discussions we had were typically the group-public discussions similar to the discussions with more experienced researchers (postdocs, PIs) in the group. I guess my manuscripts did need more rounds of polishing the text, though.
From that point of view, it is perfectly fine to have a PhD "student" coming from another field because the group/larger project needs their professional expertise. And it is also fine to expect the student to be able to get all information in their field they need (including contacting people).
There are other academic traditions that see the PhD as step of the professional qualification. And in particular where you can enter a PhD program with a fresh BSc, it doesn't make sense to expect the level of professional independence I described above.
I've been involved with rather interdisciplinary research, where different people are needed to bring several professional backgrounds (fields) together to tackle the project. It would be rather unlikely to find a professor who is expert in all those fields.
That being said, it does not make sense if there is no relation whatsoever between the PhD researcher's profession/field and the thesis on the one hand and the professor's field/expertise and the PhD project on the other hand.
But the overlap between the professor's background and the student's background may not be that large. The professor can in any way supervise the student on the general aspects of research, and can take care of their part to the project/paper.
In a more Phd-is-still-studying culture, the student may have co-supervisors that cover the (sub)fields not covered by the expertise available in their group.
It's certainly a difficult situation if you are in a lone-wolf position with your profession during your PhD. However, there are some professions where this is the typical working situation (e.g. for me as chemometrician this is certainly the case) and one may argue that the sooner one learns how to deal with this productively the better.
(This is one of the few aspects of professional life where I think the move to online seminar formats due to the COVID situation has lasting potential for improvements - while an online seminar IMHO does not offer the same networking possibilities as an offline seminar, such a student can now at least stay somewhat connected also to their profession, whereas before the decision which conference to attend may have always had the answer "application conference" before.)
BTW, I did my PhD thesis in my orginal profession (i.e. chemistry) - but the specialization into statistics/data analysis chemometrics got me to the level of what was around in the group during my Diplom thesis and when I started my PhD my professor I was not looking for guidance (as opposed to critical questions of the approach I'd chosen) in that aspect in my group any more.
So from that experience, I'd say that you can easily end up in the situation described in the question regardless of matching fields.
What is in my experience also a recipe for disaster: if the management/supervision style doesn't match the level of independence. I.e., as I said it is IMHO fine to recruit a PhD researcher because they bring certain professional expertise that is not available in the group. But that needs to be clearly communicated so the PhD researcher knows what is expected of them and it needs to be
accompanied by treating them on eye level as fully qualified professional - it doesn't mix with a supervision style that would be adequate for guided research done by a student (which could anyways not be provided without domain-specific knowledge of the supervisor).
*in my native language, the term for "Phd Student" = Doktorand doesn't have a connotation of being a student, and indeed you are not required to enroll as student when doing a PhD.