In the simplest possible terms, the atoms and molecules move with time. Heat helps this as well as the electric charge so when the molecules are not in the right places then the transistor stops functioning and the CPU ceases to be a CPU but a possible piece of artwork.
Like a lot of physical objects on the earth, they do not last forever. I have a question on the life of a CPU. How long does it degrade over use? If you use your computer the same way for the next 10 years or so, will the performance of the computer go down year after year?
Also, in this hypothetical situation, there are two new computers. One is using the CPU for conventional use, nothing more than processing important process. Another is using the CPU to the max - using the processor a lot. Will the CPU on the 2nd computer not last as long as the 1st computer?