Because they do what they are supposed to do (Keep time) and also complement the Music (Harmonically, like chords) and are capable of Solos. All at the same time. It doesn’t matter what you do, but when you listen to a Shit ton of Music, you’ll eventually end up at Jazz.
That drums can sing? The best jazz drummers have always understood this as fact. Allison Miller has even made it a core part of her artistic mission — as drummer, a composer and a bandleader, notably with her ensemble Boom Tic Boom.
Other than keeping time, the drummer is contributing emotionally to the music by controlling the tension and release. The different sounds are different tensions – not melodies. When a drummer solos – the emotion of the tension has taken over the song. He is not creating a melody. That said, there are some songs that match a tuned drum to a melodic note. That is rare.
This should convince many why it is necessary to study jazz in order to become better at rock music.