Cardinal Ratzinger / Pope Benedict XVI said he would do the job as long as the Lord gave him the strength.
And, now his strength is failing.
Last year he was visiting in South America and got up during the night, tripped over a piece of furniture and hit his head; lost some blood.
He is blind in one eye, partially deaf, has a pacemaker. And is losing weight to the extent that his tailor has to keep altering his vestments and garments.
His mind is fine, but the rest of him… not so much.
It’s a 24/7 job … no rest. Sunday is a “work day” for him; constant public appearances and talks about The Faith … catechesis. [And so are the other days.]
If you do a google search for " Benedict XVI catechesis " , you can see for yourself the unique aspects to his papacy.
For example:
annusfidei.va/content/novaevangelizatio/en/benedetto-xvi/catechesi.html
[Keep in mind that he basically is a writer … so his public work tends to be outside of his comfort zone. Bl. Pope John Paul II LOVED to travel.]
The work he has done as Pope has been grueling.
You can visit YouTube and watch the interviews he has granted with numerous radio and television hosts … he describes the housecleaning he has had to do, which must have been enormously stressful. Raymond Arroyo has described this situation in some detail; others as well, I am sure.
And there is still much work to be done and he doesn’t want to put the Church through the ordeal that it has experienced by the final illnesses of previous popes.