Why Are There Only Specific Chess Openings, But Not Chess Middlegames or Endgames?

We all know that there are specific internationally-recognised standard chess openings like the King's Pawn and Ruy Lopez and thousands of other openings... But why are there not specific middlegames and endgames like, for example, the Ruy Lopez Middlegame which will be a continuation of the Ruy Lopez Opening? I think that specific middlegames and endgames will help chess players, especially beginners to grab chess concepts, traps and tactics... Your thoughts...

There are books like the Encyclopedia of Chess Combinations (Middlegames) and Encyclopedia of Chess Endings. However, there are a lot of different positions that can occur, so other than a few named and common positions, there's not likely to be much out there that isn't more general in scope.

@1

"thousands of other openings" ++ ECO recognises 500: from A00 to E99.

"specific middlegames and endgames"
++ There are specific endgames: Rook endings, Queen endings, Knight endings, Bishop endings (same color and opposite colored bishops), Bishop vs. Knight endings, Pawn endings, Queen vs. Rook endings, Rook vs. Knight endings, Rook vs. Bishop endings etc.

The sheer number of possible positions in the middlegame and endgame phases makes it impractical to categorize them as precisely as openings.

@Unrated