It's not a gigabit interface. Fitting one would be pointless.
The Broadcom chip is designed to support the pvr functions, namely dual HD recordings and replay of existing recordings at the same time. External access will be a low priority service. You wouldn't expect the box to stop recording or playback (it's main functions).
Using the Custom Firmware, you can also stream different recordings to several remote locations, without affecting the main functions.
Gigabit speeds aren't needed for even the highest quality HD streaming that is currently available (including 4K).
A bit similar to why don't pvrs have usb 3.0 ports, same answer there is no advantage.