What? That's kind of a big generalization, man. Plenty of songs start as vocal melodies or simple chord progressions that lack melody. There are songs that have been started by an idea on drums as well and the application of chords/melody to the rhythmic structure that they create. Also, some times the vocalist is the one who writes the initial guitar parts and teaches them to the guitarist so they can build from there. This isn't meant as any sort of disrespect or anything though, just saying.
Didn't say that doesn't happen. But as a musician that's been in a band it almost always comes down to having guitar rhythms first. The drummer can't just start jamming and expect everyone to write off them. Not to mention drummers can't create harmonies or melodies, just beats. And vocalists, well most of the time they're on their own somewhere writing nonsense.