Comment by msgodel

Assembly isn't free, either an engineer or the PCB fabricator has to put that together. Also the design isn't free and it's certainly not necessarily going to match the behavior of the device on the other side.

But your laptop's Ethernet adapter comes free with your laptop (both in terms of money and waiting to get it since it's already on your desk) and possibly even more importantly you know the laptop manufacturer and users have QAed it for you so it's absolutely going to behave the way you expect which is important when the device you're designing isn't behaving.