It’s not really about the cost of the jack, moreso about the aesthetic and the ability for water to get in, because the 3.5mm barrel jack was never really intended to be on something you’d worry about getting wet. At least not at a time when waterproof ratings were a thing.

You’re talking 163mm^3 of void space inside the phone just for the barrel plug itself, plus the enclosure around it, spring load mechanisms, and housing to sit on the board. A board that also has to change position or shape to accommodate the deep round plug where it can’t exist.

Honestly I’m really surprised phones moved to 3.5mm and didn’t try to team up with laptops to keep 2.5mm the norm on those platforms, or some other plug. Had they stuck with it it probably would’ve won and also made its way to game controllers.

But there’s really no need to when Bluetooth exists and is good enough for the vast majority of consumers, and that’s all that really matters.