I've noticed that when looking at the different sets, cards that would normally be found as promos in "bundle" products and promo packs are considered part of the "main" set, whereas something like the Buy-a-Box promos are considered part of the "Collectors" set. Would it not make more sense for all promos to be part of the "Collectors" set?
Example: Triskaidekaphile (Bundle promo) is currently categorized under the "main" Midnight Hunt list.
Triskaidekaphile (UrzaGatherer) is part of the main as an extra. This is the way I'm dealing with buy-a-box most of the time :)
Some people prefer not to see it in the main set so there is an option to separate them in the options screen