Technology changes rapidly. Perhaps this is not as true for other exchanges such as history, chess, or the English language. However, it is true for Stack Overflow. Many times, older questions have answers which are no longer accurate.
Removing the accepted answer checkmark after 2 years will allow the question to become under review by the person who originally posed it. This will allow them to re-accept an answer and gain some reputation for having ensured that the answer was still applicable to the situation. This will also allow more relevant answers to be posed and accepted as answered to the situation if they are more correct than what was previously posted. It will also allow answers which contain links to be checked for link validity.
Overall, this will keep older content fresh.
"To raise new questions, new possibilities, to regard old problems from a new angle, requires creative imagination and marks real advance in science." -Albert Einstein