I don't get it either. iCloud does 50GB for $1 and 200GB for $3. OneDrive does 100GB for $2 and 1TB for $7 (which includes Office). Google does 100GB for $2 and 200GB for $3.

The cheapest Dropbox subscription is meanwhile $12 and overkill for most casual users. Maybe it isn't worth it for them to charge lower amounts on a credit card?

I worked at Dropbox briefly, and I asked this too. The answer was basically that they tried it (multiple times IIRC) and every time it ended up being a net negative on their ARR since it siphoned enough of the user base away from their other options. I think, also, the sort of user that uses less than 2TB isn't their "core" customer profile (think photographers, videographers, artists, and musicians).

I was bummed, but for a publicly traded company its a little hard to argue with the logic.

Because it creates lock-in. I have a 3.7GB folder shared with me, and I can either use the free tier or 2TB tier. Nothing in between.