Corporations tend to act on a "ain't broke, don't fix" or ABDF policy, meaning if you don't need to replace it, then don't. This was not thought up by engineers, but by the accountants. When you take an airplane out of service to change something that isn't mission critical, in this case something that doesn't help a plane full of live humans take off, fly, and land with everyone safe and intact, then it's unjustifiably expensive, much cheaper to use those mass produced yet proprietary headphones. Besides, people tend to have their phones, tablets, and computers with them on flights and their own headphones, so it wouldn't make much sense to change the in-flight entertainment system, you can do that when you retire those old planes and buy new ones.