Because wireless headphones work with all BT devices and cost hundreds of dollars less.
They are also convenient to use while commuting and while working out, which is basically 95% of my time using headphones with my phone. Ambient noise while walking, riding a bus, or being in a gym is generally too noisy to prioritize audio fidelity anyway. That's not what listening to music on my portable phone is for. I have better headphones for my desk.