Because you can add it via a USB-Ethernet dongle just like I do on my real A4000.
A simple PHY + Magnetic jack offers less complex interfacing and better latency. I have my daults on USB + dongle speeds but it might be ok.
It would be neat to be able to boot both the FPGA .bit files and .adf/.hdf files directly from network. No need for flashmemory intermediate storage.