The original question was why don't people who buy expensive things also buy expensive audio gear, not why are those people not audiophiles. I think the fact that they're not audiophiles or music lovers (or both) is adequately explained by the amount of time these pursuits take. Just like any hobby well done, it's time consuming and most people with a fair amount of money don't have time for too many well done hobbies.
I think the reason these people don't buy high-end / expensive audio systems is that the main impetus for owning other high-end things without being avidly into the hobby is status, and there isn't much status to be had by owning a high-end audio system. How many non-audiophiles have you ever really impressed with your system? Not just, "Wow, that sounds great" but a reaction that shows you really got their interest for even, say, 15 minutes. I think this is one of the reasons HT is so much more popular - it is easy to truly grab somebody's attention with even a halfway decent HT setup, even if they're not into it themselves. Super Bowl at my house on the new big screen and surround sound! A HT setup does have a fair amount of status associated with it, a high-end audio system just doesn't.