because typically people buy cheaper stuff because it's cheaper.
amd isn't better than intel for gaming, which is what the majority of the custom pc market is, if you want a cheap ryzen cpu for compiling shit sure it makes sense, but if you still want the best for gaming you'll typically buy intel.
the only objection are europeons who get a holocaust tax for intel and amd is a much cheaper option, it still doesn't eliminate the fact that intel is overall dominating the higher end market. the reason amd is doing so well is historically people buy i5s or midrange cpus to match with their higher end graphics cards.
i5s are being btfo by amd right now because they're cheaper, have more cores, better temps and are more energy efficient. the average linus tech tips viewer and /g/ consumerist is 16-25 anyway so they probably don't make very much money so a cheaper option is more attractive to them.