Simplest explanation of all - for the same reason why he carved the 'Galleon' stone, or the carving on the chimney of the house.
He liked to do it.
Amen to what Dave posted as well, if these stone maps are the genuine article, then why does it seem that they are leading every seeker to a different spot? They are vague enough that you can fit them to quite a few different places, some not even in AZ. Shouldn't they be leading every seeker to the same exact spot, or very nearly so?
I won't try to crush anyone's dream, if you believe in them or anyone else for that matter, please by all means do use them for your quest. I wish you the best of luck and hope you will return here and make me 'eat my words' - I would be quite happy to be the first to congratulate you. That said, I do hope that anyone considering using the stone maps as a guide to treasure, will keep in mind that it is quite possible they don't lead you to anything. And go prepared for the desert, don't end up another exercise for search and rescue.
Please do continue;