isn't there a brake light power feed in the cabin?
Yes, but like most modern cars, tapping into the brake light feed near the pedal can affect the ABS. The list of cars that they recommend you do not tap into the brake pedal is growing for the P3, and the most common vehicle is Ford.
Whilst you could probably find the feed in the cabin, finding the correct wire is like finding a needle in a hay stack. On the other hand, finding the brake light feed at the rear of the vehicle is simple.
There are more wires fed to the rear of my FG XR6 than a Ranger, but it was still simpler to find the brake light at the rear of the vehicle than perform contortionist acts under the dashboard trying to find the correct wire.