Because economic output always builds on previous output. That is the definition of an exponential function, and to reverse those effects you need to take the log.

It's easy to see this - just look at the early years, and notice the curve - it's an almost exact exponential curve. Graphing an exponential curve teaches you very little about the underlying data.

A logarithmic view would be great for showing when the population and output stalled or increased faster, but the purpose of this graph is to show that the fact that the increase is dramatically exponential, and at what point in the curve we are right now.

Ideally both would be shown--but just showing the log graph would be misleading in this context.

I agree. I think that the point of this graph is to convey exponential growth, and simply that. It's not meant to be used "seriously," which'd make a logarithmic graph more suitable.

Ars is correct on this one (and also resdirector, the topvoted comment right now)- the issue is not money inflation, it's the simple fact that compounding growth will be exponential, always. So if your point is to show that growth rates increased in recent centuries, then you need to show growth rates, or show this but using a log scale.