Yes they are.
Prebuilts are anything that you as the user don't have to build yourself. Sure, a "customized prebuilt" might be a better title, but it's still a prebuilt either way. Prebuilts aren't crap; you can get the same components (depending on where you buy from) in a prebuilt as you would in a computer you build yourself, you will just end up paying a bit more for item markup and their building feesOf, relating to, or constituting a structure or a portion of a structure that is constructed or assembled before being transported to its site of installation; prefabricated