As someone who regularly translates things I don't like, I can only say: it doesn't have to make sense.
People do much more nonsensical things than that in real life, so why throw stones at such a harmless activity that only concerns us?
"Dumb"? Why, I think so, but any closer examination of one's life will turn up lots of things that are pretty dumb, too. The OP's included.
In my own case, I normally keep translating series I originally liked, but only if they grow so outrageous that I start thinking, "Just where is this guy going to take his nonsense?" That I'm still doing Suicide Island and The moon is beautiful tonight, but first, die is thanks to that.
Heck, I've picked up projects just out of spite. You can call it scratching an itch: it's unpleasant, but the scratching itself is good. People derive enjoyment from the darndest things.
Other people have other reasons:
i. As a favour to a friend. I did it once. People hold their breaths to oblige someone they hold in esteem in real life, so why should that surprise anyone?
ii. Because it's easy. That happens sometimes when you're caught up with something and have the spare time. If a series is easy to do and there's no backlog, inertia kicks into action. "Eh, this takes me 30 minutes, why not?"
iii. Taking one for the team. Sometimes you don't like something, but your team members do, and you scratch each other's backs. Unless you're a One-Man-Army who only works alone, scanlation is quintessentially teamwork, and making concessions helps keeping the environment healthy.
"Completionism" is just one reason - a perfectly legitimate one - for scanlating a series one dislikes. But it's far from the only one. I think I know only one person that lets that play a significant role in their hobby.
Don't make blanket judgements on people. That's pretty dumb, too.
