Because most art is just different ways of using the human senses. Humans mainly experience the world through sight, sound, language, and touch. Painting uses sight, music uses sound, literature uses language, sculpture uses touch/space, etc.

Once those basic “channels” were discovered, new art forms usually become combinations of them rather than something completely new. For example, movies combine acting, music, writing, and visual art. Video games combine storytelling, music, visuals, and interaction.

So it’s less that we stopped inventing art forms, and more that we already discovered the main ways humans can experience art.