From the perspective of an Epidemiologist, with some published meta-analysis experience:
Must is a very strong word. Some people have pointed to where, for example, the Cochrane Collaboration requires it, and PRISMA might not, but generally speaking I've never really encountered a situation where having a single author on a meta-analysis was a substantial barrier to publishing a review.
On the other hand, I would say that a meta-analysis or systematic review should have a second person on the study team. Rarely is the literature being reviewed so clear, so well-laid out and so utterly free of ambiguity that a single person can read, digest and abstract the literature without making any judgement calls. Without having any papers where they search and simply cannot find what they're looking for. Without hitting that one paper they simply cannot make heads or tails of. There should be someone there to double-check your work, or a sample of it at least, to make sure what you described as the system in your paper and what you actually did match up. To look over those papers you've set aside in the "Problem" pile to see if they can see things you don't.
I leave whether or not that person should be an author as an exercise to the reader.
I know some colleagues who essentially begin all reviews with a parallel, blinded double-abstraction of the papers once they've been found - or even begin all the way at the search being carried out twice. I don't know that I'd go that far, but it is extremely helpful to have someone to double-check your decision making against. Despite being careful, reading closely and reviewing my own decisions, I have yet to work on a meta-analysis where I haven't been glad to have a second reviewer (or to be said reviewer).