On most of those sites (reddit being a partial exception) curation is not done by user votes. It is done by the companies themselves, partially via algorithms and partially via paid human curators/moderators) all of whom are most assuredly doing it for financial compensation.

A decentralized blockchain is fundamentally different. It is users playing the role normally performed by the company/operator in performing the curation. At scale, regardless of who is doing it and whether they are a company or not, this is a task which requires resources.

Once you recognize that (some) Steem users are performing the same function as paid company employees at centralized social sites the idea of paying users for their work is not at all perverse or surprising, but it does require a bit of a broader perspective to get ones head around.

Museum curators aren't paid for curating particular additions to collections

Museums are one particular non-profit model generally funded by tax dollars and/or a foundation. Another model is art galleries where curators are definitely paid for performance. If the gallery doesn't generate sales based on their curation, they go out of business or get fired.