@Springfairy556 - Yes, I was going to mention the Loves Outdoors trait as well. Some of my sims will prefer to sit outside in the rain or under three feet of snow to enjoy their steaming plate of goopy whatever. In some households I have to put the outdoor seating away in the winter because I still like sims having that trait to keep it.
But back on the single sim, if the chair comfort factors aren't what's driving her to do that, if you can you might try locking her in the kitchen (or locking her out of the other rooms with chairs) temporarily a couple of times until she realizes that there are viable seating choices right in front of her.