I would guess that the traders who hold millions worth of tether have a good business relationship with Bitfinex and have more information than is publicly available. And that information seems to make Tether more trustworthy. A reason why they don’t want to make their bank statements public is that their bank fears being cut off from USD clearing once it gets known that they are handling Bitfinex funds. So there are actually valid reasons to be secretive and the unsatisfactory level of public information is a side effect of the whole regulatory circus the financial sector is burdened with.