The difference I believe is how the warranty works.Those prebuilt PCs have the same DIY components as the ones he sells unless he plans on only selling Dell or HP machines, or the NUCs which are now manufactured by Asus. The Dell and HP machines also tend to be far worse than DIY builds these days. So I don't know what the alternative for a PC store owner would realistically be. Maybe become some kind of independent Mac store? Only sell laptops?
In a prebuilt PC done by the store if anything goes wrong the issue is with the store to fix it.
In a Dell / HP machine if anything goes wrong you have to contact Dell / HP to fix it.
From what I understood the store owner doesn’t want to have more headaches with faulty hardware.
It is not a matter of what is better or not but what give a less headache to the store owner.