Or because they had to do a test launch and launching a block of concert would be boring. Why not do something that gets some more media attention and thus exposes far more people and kids to Spaceflight.
But its defiantly just because of his ego, not thought went into it beyond then that.
Why is a car housed atop the third rocket? You can see it around 4 minutes after the housing is jettisoned - https://youtu.be/su9EVeHqizY?t=239
(I was just about to post this question when I decided to just look it up - I vaguely remember hearing something about this - https://en.wikipedia.org/wiki/Elon_Musk%27s_Tesla_Roadster )