Batteries are just one of the many issues when headphone jacks are removed. (Also you can totally use wireless/usb-c headphones with phones with jacks, but that's something hopefully people realize.)
"an entire USB sound card" is literally a single piece of silicon (plus some passive components like resistors and capacitors) that can cost less than 10 cents to manufacture. Not exactly a huge burden here.
Additionally, unlike a 3.5mm jack, you don't need to deal with a 'sound card' that can handle almost anything you can plug into it. You get to tailor your headphone/headset drivers specifically to your attached devices. The general-purpose device that can handle anything you can throw at it costs much more.
Except for the not-so-minor issue for us Android users: The default gain on Apples adapter is far too low for most proper headphones, and Android doesn't touch the gain and just relies on software-mixer for volume control. This results in a far too low volume. On Apple and Windows devices on the other hand, they are excellent.