He stayed long enough to tie up many loose ends from his work with his predecessor. Paper Shredder #23's warranty just expired.
That and the thing with the butler. His mental capacity is in question. Who can you trust to run things in your blackout periods? No one.
With a Papal Passport in retirement, he is untouchable. No doubt he wants to spend his final days in a monastery in Germany or be in the same old age home as his brother.
The bottom line for all to consider including himself - he is only human.