Because we can continue to learn until we are gone.
And, learning and putting what we learn into practice is a fool-proof way to grow.
Practice is the operating word here, because learning, without practice and with no intent to use it, does not take anyone far.
Continue repeating your lesson until you have learned and master what you learn.
Then you no longer have to worry about learning what you have already learned.
And, your mind becomes free to learn something new.