Because it's meant for ultra-slim devices that will be docked on Thunderbolt 99% of the time and that foldable RJ-45 jack is for the "in case of emergency break glass" scenarios, that 1% of the time when you need to patch into a server physically without wasting time looking for a dongle, not for you to constantly plug/unplug ethernet cables in your laptop.

If your uses case requires you to constantly plug/unplug ethernet cables in your laptop then you need a workstation class laptop with a full sized RJ-45 jack, not a sleek thin and light.