Nearly every economic action you take can be viewed as an investment or bet. Getting up in the morning and going to work represents a bet that your compensation will be worth the time spent. Sending your kids to school/college is an even more obvious example. For self-employed people all of this gets much more literal: every job involves a tradeoff of time and resources that could be spent on different projects. Even open-source hobby projects are can be a time-investment in building your resume. These "bets" have counterparties as well: for example, your employer can afford to negotiate much harder than you can on salary, because you need a job and health insurance more than they need you.
I also agree that this is a simplified model. But its simplicity is what makes it elegant: you can see the effect in a model that lacks all of the complexity of real-world economic activity. In the real world the "bets" are more complicated and the odds more variable, but you can't just claim "this effect must go away" without articulating a clear reason that it would.
The reasonable point you do make is that in our current economy the "pie" isn't fixed: new wealth is being created all the time, and this is one reason we don't collapse into permanent inequality the way this model does. This doesn't negate the model, however, it just means there is something counteracting it. Unfortunately the fear is that in the future (or perhaps even the present) new wealth creation will no longer keep up with this underlying concentrating effect, and we'd better think hard about what to do then.
