Because while it's only an "ok" item relative to those, it's something that drops super early in the game and can scale very well into lategame, meaning if you find it, you are set until you can actually farm one of those other swords. It's supposed to be a super lucky drop that gives you a great head start, not a "lets farm this early in the league and not have to worry about progression again until you can beat the end-game bosses" drop. Lowering it's drop rate enough so that it's now longer something you wanna just farm and grind away returns it back to that super lucky drop status. If it happens to drop for you, it's awesome and you can be set, but it's also not really worth spending the time to grind early league and skipping the entire progression route.