Whaler,
Two reasons:
First, the gauge response will be VERY slow with 1/8" tubing.
Second, and more importantly, safety. Copper tube will work harden from vibration and eventually break, spraying hot
oil all over your very sensitive area.
Also, if you use a fuel press gauge, use an isolater in the engine compartment. You don't want to trust even braided line piping fuel into the cockpit.
Later,