Are there any sources of massive transposition tables? Do transposition databases exist the way tablebases do? Would an engine be able to take advantage of a pre-calculated TD? Is there a benefit to players when studying if they had access to pre-calculated positions?

What I am imagining is a sort of Wikipedia for chess positions. Rather than focusing on perfect endgames or history of play, the emphasis would be on the position itself rather than the moves leading up to it. Users would then “kibitz” the individual positions, with popular ones rising to the top based on visits or appearance in games.

Does something like this already exist? I’m aware this is similar to some of the features of “Let’s Check” from Chessbase. I’m thinking open, like tablebases are.

And on that note, are there any proprietary table bases?
What you're talking about is basically opening books. If an engine has access to hard-set evaluations of common positions fairly early on in the game, then it can play better.

But having a pre-made transposition table/database of positions occurring far after the opening isn't useful, since the chance of reaching any particular position late in the game is extremely low. Unless this position is part of theory, in which case it would be in an opening book.