Here the big difference between movies and TV shows. A movie has about 120 minutes of film and a 23 episode season of an hour long TV show has 966 minutes, thats eight times as much film. Thats not even getting into the multiple seasons TV shows have compared to movie sequels. Its not logistically possible for TV shows to produce that much film without a certain number of characters. A successful TV show is either a true ensemble or has a lead(s) and supporting characters. Flash, Supergirl and Arrow all use the single lead with supporting characters format. Grant, Melissa, and Stephen are three of the most charismatic leads currently on TV but a great lead still needs other characters to make the show work. Those leads need characters to interact with in meaningful ways in order to move the leads story along. In my opinion it actually helps the leads story to have strong fleshed out supporting characters.