Those top grossing numbers are deeply affected by the entertainment landscapes of their eras. Movies played for much longer in the past and there were fewer other entertainment options. So it is not apples to apples.
And? Population and demographics have changed as well of course. What does it matter? We're not comparing movies based on which had the biggest impact on the people of their time, we're talking about business. Of course competition in the landscape of entertainment is going to affect that, that's the whole point.
What about if the price of apples went up you could still be making the same amount of money as when they were cheaper...and you need less people to buy your apples. I can almost guarantee that a smaller number of people saw Avatar in theaters than Gone with the Wind.
A movie ticket, on average, cost $.23 in 1939. Adjusting for inflation, thats $3.72 today. I think I paid $15 for Avatar 3D.