I have a friend who lived in the USSR and I asked him the same question. In a world where everyone is paid the same, why become a physicist when you could just be a cab driver? His answer was pretty much what you'd expect: different people aspire to do different things. It's not about the money (physicists were working multiple jobs to make ends meet), it was about what they wanted to do with their lives.
And how did that work out for the USSR? An unfortunately small minority of people are driven by enough passion to ignore how they are being screwed by society (sacrifice all of your 20s to go through med school to get the same compensation as a high school dropout that sweeps floors).
But not everyone had the same amount of privileges. People in politics often had bigger apartments, for example. So if you wanted "more" in a communist state, you went into politics.
