It's only taboo for people who know what they are doing. If you don't have the skill, it's understandable. If you don't have the "time", that seems more like an excuse.
As many have mentioned, you get more power/speed for your buck if you build your own. That said, I always recommend my sisters and parents buy prebuilts, because I don't live near them, and I know if it breaks, I'm tech support, and sometimes doing that over the phone isn't easy - especially if there's a suspected hardware problem. So since building your own can be finicky at times, I find them the best deal I can with the major brands and push them that direction. They aren't gamers either and often you can actually find a good deal on a low-end PC, so long as you don't click to upgrade any of the default options, that is cheaper than building your own. Since they are just checking email and and social media, that works just fine for them.
But if you know what are you doing and are a gamer, and you buy a big-brand prebuilt anyway, most folks would look at that as either lazy or having more dollars than sense. So that's the only time where the "taboo" comes in.