Answer

At the moment it is about 80 years,  at least in Australia, and it varies around the world, not really 100. We are really just rounding up to make it a nice number that comes close to our decimal way of counting. And it wasn't always like that. We, in our current evolution, just happen to be living that long now. In the past we lived a lot shorter. At one point average life spans were a lot lower than that, around 20-30 years.  

This is a very deep question the answer can be long and winding but if I can just stick with your question WHY, then the short answer is because we evolved that way. 

What I mean by this is our life spans as human beings were determined by that of our ancestors (primates, mammals, reptiles, fish, etc). Their life spans were the starting points of the next generation of species that arose afterwards. 

For example our near ancestors,  belonging to the homo genus and the primates lived no more than say 20 to 50 years. Since we inherited their genes, we can't really expect to be living 500 or 5000 years all of a sudden. 

So if we are to live 1000 years, then it must be because there are survival advantages to doing so. If we are to live that long then it will be a slow process of evolution that moves from generation to generation.But the direction of evolution is only so, as long as the process of natural selection favours it.

Do you have a different answer that might help the questioner? How would you answer this question?


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