Why did the first screen actors use exaggerated gestures, emphatic expressions, and the mouthing of words to bring their characters to life?

The first screen actors used exaggerated gestures, emphatic expressions, and the mouthing of words to bring their characters to life because they adopted the acting style of the 19th century theater.

Prior to acting on screen, only theater or stage play were the means to watch a play. When filmmakers were looking for screen actors, only theater rejects or amateurs were willing to become part of an emerging industry. Their lack of experience and lack of acting workshop made them adopt the acting style that was very popular in the 19th century theater and adapt them on screen.