So, I looked on the lichess analysis board and played around with it for a while, and I couldn't find any reason why 2...c5 isn't considered an accurate response to 1 d4 d5 2 c4. Does anyone know why this is?
After 3. cxd5, any sensical black response is met with 4. dxc5. The time it takes for black to regain those pawns is lost on white's development, e.g. 4... e6 5. e4! exd5 6. Bb5+ Nc6 (or Bd7 7. Bxd7 Qxd7 8. Nd2) 7. Nc3.
White's opened up the center - favorable because of black's daft, dithering development - and obtains a game with winning chances.