Why can evolution not take place in the lifetime of one individual?

Because we adapt to long term environmental conditions only where it is required for us to survive as a species with small changes each generation and a generation can be a decade for humans. We can see minor changes going back to the 40's/50's.

It was once proposed, by Jean-Baptiste Lamarck, that individual organisms might, through habit, acquire genetic traits that they would then pass on to offspring, so that they would genetically inherit the products of their ancestor's habit. Giraffes might for instance inherit a longer neck because of the habit of continuous stretching-of-the-neck that their parents developed to get at higher leaves.

But no evidence was ever found of this inheritance of acquired traits (although epigenetics do show something similar to this), and the hypothesis had to be abandoned in favour of Charles Darwin's thesis: that populations evolve because of natural selection acting on the inherited traits of individuals within the population; traits that only vary as they are passed from parent to offspring.