Its actually quite simple...you are paying almost completely for the research and development of the components, rather than the components them self.
Lets make an example, You have $2000 budget every 3 years for PC.
With that amount you can purchase,
1 top of the line machine every three years
OR
1 med/high end machines every 1.5 years.
The difference in playability from a high end to mid/high end machine is next to zero. However, as you use computer components they tend to wear out, and over time slow down and even go bad. So, even though you lose out on some speed on the first 1.5 years, at the start of the second year the new computer will blow the old top of the line model away.
The only reason to ever buy a top of the line machine is if you have no budget issues.
And if that is the case. Stop being greedy, and give some money to charity instead.
So the moral of the story is, there is never a reason to buy top of the line gaming computers.