The answer is literally always “It will allow me to pay my bills. This won’t be a giddy, young-love crush of a hobby for me. Hire me and I’ll just do good work, not cheerlead.”
The question is a synecdoche for “How awesome do you think we are, and so how much will we be able to coast on our respect for you as a result?”
Not true. I can think of a bevy of reasons that I'd be excited for a role that have nothing to do with being a cheerleader. For example, working in the banking industry for many years, I was tied down with a very rigid, sometimes antiquated tech stack. I started applying to smaller companies because I was excited about the prospect of having more autonomy and being exposed to a more modern tech stack.
The question is a synecdoche for “How awesome do you think we are, and so how much will we be able to coast on our respect for you as a result?”