Because we try to ensure that we are as creative as possible ie choruses do not just come on as general crowds ? that everyone has fully developed characters with detailed relationships and identifiable ?stage business?. When someone is missing it changes the whole look of a show and the balance of lines/tableaux etc. Also health and safety ? we have to ensure everyone is at the Theatre the whole time to listen to notes, that may arise/become apparent throughout the week in this new space.
Also and maybe most importantly, it would be hugely unfair to other company members if their group members/partners weren?t always there, as it would greatly affect the work they were doing and had rehearsed throughout the term.