There's plenty of use cases for small things which don't need any sorts of speeds, where you might as well have used a 115200 baud serial connection but etherne

There's plenty of use cases for small things which don't need any sorts of speeds, where you might as well have used a 115200 baud serial connection but ethernet is more useful. Designing electronics for 10Mbit/s is infinitely easier and cheaper than designing electronics for 100Mbit/s, so if you don't need 100Mbit/s, why would you spend the extra effort and expense?