ECR (engine control room) tours are occasionally possible.
Engine room tours are extrememly rare.
Today's modern engine rooms can be quite dangerous, with hot steam pipes to burn you, slippery catwalks, high voltage, and very loud machines.
But the most dangerous - and the main reason why you cannot tour - is fire suppression. Most engine room fire suppression systems are CO2 and HALON. If they are activated and you happen to be in the engine room at the time, you are dead. The cruise lines want an absolute minimum number of people in the engine room at any given time.