I once wrote a poem:

The work of building will take ten thousand lifetimes.
But dear one, look –
that work has been achieved ten thousand lives ago.

This is speaking from the point of view of the ultimate dimension. Do you need to become a Buddha? Do you need to run after enlightenment? The wave does not have to seek to become water – she is water, right here and now. In the same way, you are already nirvana, you are already a Buddha; you are already what you want to become. What is essential is to enter the path of practice in order to realize this truth and help others realize it too.

With his limitless life span, the Buddha has unbounded capacity to help living beings throughout space and time, in all the realms of existence. But he plays at the pretense of coming and going, being born and passing into nirvana, as a kind of skillful means to encourage living beings to enter the path of practice.