The Problem ist people only see this from their own perspective. It is totally fine to have an prebuilt PC everything has advanatges and disadvantages. I know many people who have prebuilt ones out of different reasons.
The biggest con of a prebuilt computer is cheap hardware in the important parts. Most bigger prebuilt Companies tend to go bigger on smth they can advertise like OMG this PC has 20 GHz then put that CPU on a cheap msi Mainboard with little Features and so on.
I think this Opinion is a ghost from the past where the choice of those "shitty Hardware" like Elitegroup Mainboards or Mainboards with SIS or Via / nForce Chipsets where much bigger.