Why are there so many production companies involved in a film?

Contemporary film-making often involves many different production companies mostly because of the complexity of the overall task. To continue to create movies at a high quality-level, contemporary film-makers make intentional use of experts in every related dimension of film-making -- lights, sound, effects, etc., as well as location-finding, supplies, and other support-needs -- and often collaborate with overview-production teams while doing so.