There are many reasons for this. First, there's no guarantee that the temperature will be comfortable (it could get too humid or be too breezy). Inside the restaurant, you needn't be concerned about such things. Outside, flies (or worse) can get into your food. This could also happen inside, but the likelihood is much less (though the chances of seeing a cockroach are much greater indoors). Then we have the problem of traffic noise and smokers.
A few months ago, I went to a Friday's in Wisconsin and was quite surprised when the maitre d' said, "Smoking or non?". I was like, "Excuse me, my good man, but I wasn't aware that I had teleported to 1973. Now, please excuse me while I head out to find a young boy named Barack. Legend foretells that one day, he will change the world. Oh, and do you know any places around here that sell frankencense and/or myrrh?"
There's also the problem of sunlight when eating outside. Course, they usually have those umbrellas, but that's sometime not enough and I don't like to have to wear sunglasses while munching on some fingers. In addition, I love sitting in booths which are generally only available within the restaurant's walls.
Quick comment about eating: I only ever put salt on one food item: corn on the cob. In my mind, if you have to put salt on something for it to taste good, why are you bothering eating it?