How come this prebuilt is cheaper, but equal specs to my custom?
Prebuilts always have a bunch of bloatware software crap installed on them, they probably get a bunch of kickbacks from that.
They probably work out deals directly with the part manufacturers, buying straight from them in bulk, rather than a middleman.
They may be just selling at or near cost, so they can just get rid of old models to make room for newer models that everyone wants to buy
They usual cut corners and use cheaper stuff on things like RAM, PSU, and mobo.