Why does Max not talk in Ruby and Max

Ruby and Max is a film starring Emma Stone and Joaquin Phoenix.

Ruby and Max is a 2019 American drama film directed by Jonathan Dayton and Valerie Faris. The film stars Emma Stone as Ruby, an aspiring actress, and Joaquin Phoenix as Max, her older brother. The movie also stars Steve Carell, Toni Collette, Rosie Perez, Bill Pullman, Danny DeVito, John C. Reilly and Rumer Willis.

Max has cerebral palsy which affects his speech. He does not speak throughout the entire movie. However in the end credits he has one line: “I’m not talking now.”