They can, it's just that the trade-off isn't worth appeasing a few HN readers.
>The reason earbud companies use non-replaceable rechargeable batteries is simple: It makes the earbuds smaller. Earbud buyers generally prefer more compact devices, but that means the earbuds have less room inside for all the necessary components. Designers need to cram a Bluetooth chip and processor, an antenna, a battery, drivers, controls, and microphones into something that’s often the size of a thimble. Replaceable battery compartments require more earbud real estate, and in a competitive field where tiny is currently king, companies don’t want to risk their earbuds being flops by making them bigger.
>The reason earbud companies use non-replaceable rechargeable batteries is simple: It makes the earbuds smaller. Earbud buyers generally prefer more compact devices, but that means the earbuds have less room inside for all the necessary components. Designers need to cram a Bluetooth chip and processor, an antenna, a battery, drivers, controls, and microphones into something that’s often the size of a thimble. Replaceable battery compartments require more earbud real estate, and in a competitive field where tiny is currently king, companies don’t want to risk their earbuds being flops by making them bigger.
https://www.nytimes.com/wirecutter/blog/your-wireless-earbud...