It’s only the strata manager’s job if the owners corp via the committee makes it so. In the majority of cases, the committee mambers may feel that they lack the skills and/or experience to do that properly themselves and delegate the job to the SM.
In any case, whatever is negotiated has to be approved by the committee or the owners corp as a whole so there is plenty of scope for a committee member to undertake this themselves, knowing that they don’t have the authority to sign off on a deal anyway.
But in short, it’s up to the committee to decide who does the negotiating and that job would often fall to the SM as they have the skills and experience (or should have).