silly question is silly.
When you buy a pc:
it comes preloaded with shit you don't want.
you can only purchase the models they make, with specific specs.
you pay for a name badge
when you build:
you choose all the parts
you install the OS yourself, and it wont be full of trial crap
you dont pay for their labor time, since you build it yourself
The whole is always worth more than the sum of it's parts when you buy, but you can build the same PC for alot less.