Prayers fail because there's no God – so there's no one to grant the wishes and desires of those praying. It's quite straightforward when you think about it.
Take Lourdes for example. One of the most holy places in the Christian faith. Multitudes of people flock to Lourdes every year because they are told by the Catholic Church that this is the place where miracles can happen. This is place where the sick can be cured and the afflicted healed. How many devout people have appealed to God in this place to cure them of their ailments? Millions? Tens of millions? Hundreds of millions? Who knows? According to Wikipedia, Lourdes expects up to 5 million pilgrims and tourists every year. That's a lot of people and by inference, a lot of prayers.
How many permanently wheelchair-bound people have left this beautiful town in the the Pyrenees walking, unaided, by themselves? How many amputees have left with their limbs miraculously re-grown? How many terminally ill people have left cured of their illness?
Not surprisingly, there has never been a single verifiable incident that has seen a person with any scientifically incurable illness or condition go to Lourdes, pray to God and leave the place cured.
Not a single one. Not one, not ever.