My phone is much clunkier to use for this than a USB stick. Plus my phone can't store my whole music library (because there's too much other stuff already on it), whereas a single USB stick does it easily with plenty of room to spare.

Phones can access the whole internet. A USB stick can't stream Spotify or connect to the cloud.

Which doesn't matter to me since the music I'm talking about is music I already have on the USB stick (because it's music I've collected over years and years of buying CDs, mostly before ways of streaming music over the Internet even existed), and I mostly want to listen to that. If I want something else, I can just use XM radio.

You'll be surprised, but some places where a car can go have no phone connectivity.