I don't subscribe to the "instant gratification" idea. People of a certain age didn't "choose" delayed gratification - it was the only option that was available to most of us. When we had the opportunity for instant gratification, we jumped on it.
These days, layouts are larger, our expectations are higher, and most of us have more demands on our time. Unless one is a skilled and prolific builder and can spend 40 hours a week on their hobby, there's just too much to do. Most folks save their limited time for scratchbuilding for special/unique aspects of their layout.
As mentioned by several others, the manufacturers have determined that the small cost difference between a kit and a fully assembled model is not something that the market wants.
