Because as Dan says, it's a system, not a part of something else. In order to implement it properly you have to commit to it and it has to be core to your offense and your offense bends to it, not the other way around.
So when Payton brings in a/the guru consultant for a bit, that's just for adding a bit to his offense, not changing the core of his offense.
In culinary terms, if this Shanahan offense is a steak seasoned with salt and pepper, what Payton did was add a bit of salt and pepper to his Tilapia.