For many, yes. It's not the layout, it's what the owners want out of the experience, and for many it's the conceptualization and then the construction. When it's all done, say in two or three years, there's a period of interest, but then another idea comes up and that becomes the focus. It's the same with model kits. Some like to build them, but not necessarily have a working railroad.
To situate me in the topic, I create layouts because I like a realistic setting for the trains to run in. For me, the hobby is about running trains. Building a layout is merely a means to an end. It's not that I don't get much out of building a layout, or even from planning a new one...I just like having the setting to play my trains with.