With more context it makes sense. It isn't just smartphone batteries, but lots of consumer electronics. Phones, tablets, cameras, ebikes/scooters/cars. And other parts of the legislation are focused on battery recycling targets for long-term sustainability.

From another article on the resolution:

All electric vehicle and rechargeable industrial batteries above 2kWh will need to have a compulsory carbon footprint declaration, label, and digital passport.

The parliament also passed new targets for collecting waste and recovering materials from old batteries.

They're targeting batteries (first) because they use so much lithium and other relatively rare metals, and having so many batteries up in landfills is not only terrible pollution when they leech into water and stuff, but it's just not compatible with our current and foreseeable dependence on lithium battery tech.

It's the only component that would wear out no matter how you use the phone.

Because it's a consumable that renders the entire device useless when it goes bad.