Why are celebrities so accurate in Mandarin and have no dialect accent?

The State Administration of Radio, Film and Television clearly stipulates that "announcers and program hosts of national and provincial radio stations and television stations should have a Mandarin level of Class A. Announcers and program hosts of other radio stations and television stations The Mandarin requirements for hosts shall be in accordance with the regulations of the State Administration of Radio, Film and Television. Actors and dubbing actors in dramas, movies, TV series, radio dramas, and teachers and students majoring in broadcasting, hosting and film and television performances must have a Mandarin level of no less than Level 1. "As far as I know, many city (state)-level radio and television stations have regarded "the Mandarin proficiency reaching Class A" as the industry entry threshold. Therefore, if the actors in the movies and TV shows you see on TV are not good at Mandarin, they will usually find someone else to dub them.