Why include a headphone jack?
It seems weird to extol the virtues of a piece of nearly 140-year-old technology in this day and age, but the reason why it’s persisted this long is that… it works. It’s a solved issue: not only is a TRRS plug cheap, but it’s durable, small, and high-quality. It can support inexpensive headphones, and it can support the best headphones—all with one universal standard. Not only that, but it’s an easy way to enable the use of microphones, as well. It’s a fantastically versatile piece of tech that hasn’t really been changed all that much since the plug was reduced in size to 3.5mm in the 1950s.
Obviously, a headphone jack isn’t the only way you can listen to music. Bluetooth headphones exist, and they work well for most people. But the problem is: they don’t work well for all people. Audiophiles probably aren’t too keen on being unable to listen to high-bitrate files, and there’s no shortage of reasons why you wouldn’t want to deal with the added hassles of wireless tech… Which is why smartphone companies have used both in tandem for so long: it offers the pickiest listeners quality that Bluetooth can’t currently touch, while enabling users who want to ditch the cable. There’s almost no reason not to have both, especially when the cost to manufacture is so cheap.
Another primitive tool, the headphone plug does its job fantastically well.