it was a decision the devs made. Not sure if they ever stated the reason.
There's a mod with which you can make them playable. Just google.
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@gamecritterytand @crinrict , I heard that the developers decided to make the pets in The Sims 4 not be controllable because they wanted the gameplay to reflect our real-life experiences with pets. In real life, pets are always a bit of a mystery in what they are thinking and what they will do next. The developers wanted that in the game as well. While I would have preferred to be able to play as the pet because it is more immersive , most Simmers wanted a more true-to-life experience .
@jpkarlsen, thanks for the info! I've never thought of dogs in TS4 needing baths as the 7th Need! Interesting!