The builders in question do it because they feel it makes up for their lack of skills of basic model building, and by adding all sorts of aftermarket goodies, it makes them feel better about their build that in their mind would other wise suck, but in reality, it just looks like when the MythBusters polished up a turd.
So in their minds, the over compensation of the aftermarket stuff will in turn make the build better,and hopefully hide all the stuff that was not done properly from people looking at their build, and in most case's , it will be overlooked, and the build and builder will get the praise.