The Sims 3 has a vast array of features already. Sometimes, certain actions are left out to keep the gameplay balanced. Autonomous bedtime story could potentially slow down the game or make the Sims' behavior too complex. For example, if Sims were to start telling bedtime stories autonomously, it would need to involve selecting the right book, the right listener (like a child Sim), and appropriate animations. This all takes development time and resources, which might have been better spent on other aspects of the game.
It could be that the developers didn't consider the bedtime story as a high - priority autonomous action. They had to manage the complexity of the game and its various features. Adding an autonomous bedtime story might have required additional programming for the Sims' behavior, dialogues, and interactions related to it. So they decided not to include it in the game.