The engineering that was done there to bury whatever is pretty impressive.
Maybe someone in the prior digs actually found the treasure and kept mum about it. Whatever was down there may no longer be.
When modern technology has a very difficult to impossible time excavating the site you have to be impressed.
A lot of thought and manpower went it.
A possibility is that the site was built to hide something but that something never made it there.
Pirates wouldn't bury treasure, in fact burying treasure doesn't make a lot of sense. Burying it on an Island that is at the water table makes no sense.
Making an elaborate chamber with flood tunnels to protect it makes no sense. It probably wasn't even possible given the land they had to work with.
Using a hundred or more workers to "hide" valuables makes no sense
What makes sense is the island is full of sink holes where trees fall in and are then decades later determined to be planks and timbers found underneath the ground
People lived on the island which explains much of what has been found there.
Tourism was likely a part of it and "salting the mine" wouldn't be out of the question
Just the concept alone of burying treasure in such an elaborate design raises red flags all day long. Did they plan on having access one day? If not, why not just throw it over the ship at sea?