It's not a taboo if the prebuilts are from GOOD companies like NZXT's BLD. Most prebuilt systems have horrible mainboards, custom GPUs and PSU's which makes them horrible for the costs.
Going to a PC build enthusiast group and saying someone should buy a prebuilt would be like going to a mechanic enthusiast group and asking where they get their oil changed... You are talking about probably one of the most basic entry point things that someone would do as they come into the hobby. It doesn't mean people don't do it (as time is always a factor for some), but the mid to hardcore people will always mock the idea anyone would offload such a basic task to someone else.
I have actually never purchased a pre-built PC but I can see why people are drawn towards it. The fact that you can customize and upgrade later is a very inviting thought. Maybe next time, I will get one.