Series start with a concept for 2-3 seasons at most, nowadays often just one. If it’s successful, writers need to come up with an extension fast, since nobody wants to wait for the next one for 5 years. Sometimes they are greenlit halfway through the screening of S1 and they want to start filming half a year later, so there’s not enough time for due process with proofreaders, test audiences etc. as you do with movies, where sequels often come several years later.