I've seen Gosu forget the Sweeper streaming with Diamonds. It's not "common sense," it's a common mistake because it's unobvious as it's a reactive item, e.g. an item that you only use in response to the enemy team doing something. For example, how often do think about buying a
before you've been ganked by Warwick?
It's asking for a quality of life change. For every other role it's not a huge problem if you forget to pick up a sweeper---you still get to place wards, but for supports with sightstone it literally means you're now wasting your trinket and you're getting punished for it because the cooldown timer on using your sweeper first begins the second you actually go back and get that sweeper. And it's a secondary purchase that you have to make every time you buy a Sightstone.
It's the first kind of shop mechanic that's more of a hindrance than convinient to the player. The UI in the shop has intentionally been made super easy because the item in League is very complex and new players wouldn't be able to understand the game if it was anything like it was in Warcraft 3's DOTA. And this is one of those situations where the item system asks you to remember an unnecessary detail that has no actual impact on your game play.