There are several reasons really. The Sierra upright isn't ideal, the offset mushroom introduces some mechanical trail to help self centring, and people think that you can just add caster (by moving the top wishbone backwards or the bottom one forward) to give more self centering.
It's not that simple.
All suspension design is a compromise and the price you pay for increasing caster in this way is that you move the contact point of the tyre (
When turning) to the outside of the wheel, eventually the contact patch can be on the sidewall. This causes "flip/flop" steering, where the steering is fine in a straight line but as you turn the wheel you reach a point where the wheel is pushed into the turn giving very odd feeling steering, almost like having a left and right switch.
I suppose the reason it's set is because it's right, that might sound a bit strange but I work on "a little knowledge" principal, which means that most people want the car to handle in a neutral way and don't have the experience or knowledge to adjust the caster and get good results. Now some builders
will have that knowledge and it's possible to make the caster adjustable by using bigger brackets and swapping spacers either side, it was my judgement that it wasn't something that was necessary for most builders and it was a potential minefield for the inexperienced.
If you are going to alter the suspension I'd recommend you use a program like "Susprog" and check your mods, it can be very unpredictable.
Cheers
Chris