Then why don't they add weather and pets?!
Well, I'm sure that they're not done creating expansion packs just yet.
Like I've said before: the pet expansions for Sims 1 and 2 were very popular, so it extremely likely that there will be such an expansion for the Sims 3, as well. However, judging from the expansions that have been released so far, there are a lot of
new ideas rolling around in their developers' minds, so they're probably going to use those up first before they go back to the 'old hat.'
Weather, on the other hand, would be problematic at the very least. Gardening has been around since the base game, and is a tried and true system. For them to introduce weather into the game, it would probably require a complete rework of how that entire skill works. Growth rates would have to be sped up, harvest quantities would have to be increased and skill gain rates would have to be adjusted to compensate for the fact that there is at least one season that this skill would not be able to be trained at all. And when you consider the slow growth rates and low harvest quantities of some of the rare and special plants to begin with, you can imagine how difficult it would be to get all of these changes balanced. At this point, weather might actually be something that just isn't possible.
However, this is off-topic, so let's try to get back on track.