That would because many strata management contracts give them the right to do so (it's a good fallback for when the office-bearers are not available).
Also, often the strata manager will be asked to chair a general meeting when the actual chair is seen to be on one side or another of a contentious issue (and their contract allows them to do that).
The problems arise when the strata manager assumes the responsibility and no one has the nerve or the knowledge to challenge them but these situations are pretty rare and, in any case, often flow from frustration at an SP's inability to make decisions and move forward.