My favorite opening is the Queen's Gambit Declined, and I will often reach positions like this
[FEN ""] 1. d4 d5 2. c4 e6 3. Nf3 Nf6 4. e3 c6 5. Nc3 Be7 6. Bd2 O-O 7. c5
Where I decide to play c5 after my rook is safe. If black goes after the c5 pawn with their pawns then I defend it with mine, and when I run this with Stockfish the pawn is indeed not captured.
The thing is, this move is generally always classified as an "inaccuracy" or "good move", but never as "excellent" or "best". To me, it feels like this move severely cramps black's dark-squared bishop as well as just gaining me a lot of space and control on the Queen's side.
In general, why is playing c5 as white in the Queen's Gambit Declined not a good idea? As black, how should I go about defending against this move?