I would argue the cost is not much higher than fast food, but the quality is arguably still a large step up. Obviously these places get a lot of points for their excellent choice of bread (be they sticks, biscuits, etc.), and there's arguably something to be said for going to a restaurant and knowing they have food you like.

If you go to these restaurants a little more regularly, you also can stack up the rewards benefits, sometimes you can get discounted gift cards, etc. All of which you will not find at a non-chain restaurant.

Which is to say, if I can get a sirloin for the cost of a Whopper, it isn't much of a mystery why people go there.