KSP gives us zero mass, zero drag, infinite flow fuel lines. No parts fail in any way, unless the design itself is flawed. And, we don't need to worry about cost. In reality, engineering Asparagus is more difficult. The needed pumps are fairly heavy and add additional points of failure. On top of that, real life engines have a much better TWR than KSP engines, so the advantages of Asparagus over simple parallel staging are not as significant. That said, the SpaceX Falcon 9 Heavy does use asparagus staging with two boosters.