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.
I know what you're saying about it happening a majority of the time in most bands, but as a musician who's been in numerous bands, toured, and taught music for a living, I can tell you from my own experience the things I said happen, do in fact happen. I've written plenty of songs that started with a vocal melody, I've also written plenty of songs that started with a drum idea that had the right feel and drive that I was going for. Also, the drummer in my current band writes a lot of the vocal harmonies and also does some backing vocals. I'm doing vocals in this particular band as well and can assure you every song that's been written for it was written by me and I taught the parts to our guitar players. I've also had songs start as basic chord changes with no melody and build from there, and even songs that I wrote on piano and transferred to guitar in order to more fit the style. Again, this isn't meant to be disrespectful by any means. I see your point; I just feel as though it was a broad generalization. Not looking to argue at all though.