Ah, yes, but if you custom build you can get more performance for less money. you pay for the convenience of it already being put together :3
My PC was a barebones prebuilt, so it already had the case, CPU (+ custom cooler), PSU, Motherboard and RAM when I bought it. I already had a GPU and storage to put in it so I just completed it from there. For some reason it was cheaper to get the barebones prebuild than it would've been to order the parts myself. That, and the fact that it came with a two year warranty, made me think "well why not save the work AND get it cheaper?"