Because of some silly idea of what makes something feel modern.
The worst case of this in cars. Steering wheel controls wouldn't need to be a thing if I could reach towards my stereo and feel where the track skip buttons are without having to take my eyes off the road.
Consider that at a critical moment the difference between moving a thumb and moving ones whole arm and hand could be the difference between life and death.
EDIT: fixed typos
For everyone else, we've had automatic cars for forever. There's also a video online of a man with no arms who learned how to drive their car without them.
Edit: changed to be specific about the one hand on the wheel.
Re: steering wheel buttons in general, it depends on how they're implemented. If you take an old idea [1] and provide tactile locating then it is easy to learn. If you can reuse a common paradigm (a knob for volume) there's no learning required. The steering wheel also is already a busy place - you have two or more stalks nearby, sometimes cruise control is part of the steering wheel buttons and even if not the amount of space means you are forced to provide a subset of functions. A stereo with tactile-marked buttons and knobs is known to most drivers via other paradigms and is easy to swap between cars.
Seriously, a rotary knob can switch focus to the next/previous UI control, so that additional push of tactile button on said rotary knob will activate the control, but they can't simply detect the rotation as 'next/previous', with the push button as 'pause/play' toggle?
Instead I have to rotate the nob and visually track the thin active element outline each time I enter or swap to the app just to get a single tactile 'forward' control. Or, I give up, and continue the process of memorizing the button location on the screen and building muscle memory through repetition to correctly press the same spot on the screen each time without looking at it. Then hope I never need to be in a different car or that nobody adjusts my seat's X/Y/Z axis to screw up that muscle memory.
I suspect it's Spotify's issue, and not Android Auto itself, but most of the Android Auto apps work that way and I don't know where the fault originates. But regardless of who can fix it, it's horrible to allow UX to be designed ignorantly when it is placed to have potential life/death impact.