You're right - it isn't realistic. It's a game...especially the first one. It tries to be a game far more than any sort of realistic simulation. Later entries in the series - like Deadly Shadows or Thief 2014 - try for more realism than is necessary.

There's a reason why people are still making fan missions for the first two Thief games today while Deadly Shadows and the 2014 Thief are ignored: Thief Gold and The Metal Age are fun to play.