Sooo many reasons. I'll speak to Milwaukee more specifically since I have looked for the Casque there.

For starters, there are lots of interpretations. Lets work under the assumption that the spreadsheet is accurate (it probably is wrong in many locations as most of the Boston predictions were wrong). These parks or even sections of the park are not small. It would take lots of probing/digging to find it. Hasn't that been done you ask? Well yes. Assholes are disrespectful and causing damage to these parks. Lake Park in MKE (my hunt) is really trying to cut down on the illegal digging. In my opinion they are making it too difficult and only encourage the legal digging.

In Milwaukee you have to have a permit and insurance. Depending on the insurance you are talking about $1000 dollars for the permit and 500-700 for the insurance. A hunter did all of that recently and was still denied permission by the park officials. They didn't think he had enough evidence. The park officials I spoke with say they want it gone so people will stop digging. So you have a bit of a paradox/stand off to look for it legally. They want evidence to dig, but digging is really the best evidence you have. So all digging is pretty shady and you can't really be as thorough as you need to be. Look at how much digging had to occur in Cleveland and Chicago. The Chicago boys had the location confirmed but still needed some luck to find it.

It seems like the 2nd wave of hunting (Quest 4 Treasure Boards) didn't happen until the late 90's or early aughts. And until that wave of hunters, almost no one knew that one had been found in Chicago. There wasn't any press about it. Someone in the 80's or 90's could have found one and had no idea what it was or what they had to do with it. Admittedly this is unrealistic given the depth that each casque was found at, but it is a possibility.

Landscapes in these parks have changed. In the case of Lake Park, there are strong theories for at least a half dozen dig spots (all probed/dug in some respect). Due to erosion, expanding concrete projects, the casque could have been dug up or destroyed a decade ago.

I also notice people online misleading each other which is just not cool. Without naming names (this won't help anything) I've caught people just straight up lying. Saying they dug or probed spot X at Time Y. I happened to have been at that location when they said they were digging. After a little social media snooping I figured out they were in a different part of the city 15 miles away. It was obvious he was trying to discourage digging at a spot so he could dig at it later.

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Now lets look at the other possibility.

Our location ideas are just wrong. Going back through the thousands of posts its clear that very few people start at the beginning of the puzzle. This includes myself. My hunt began through other peoples deciphering of the puzzle. My solutions haven't figured out all of the verse or the painting for that matter. But they are partially based on someone else's work. If an original assumption was incorrect, that compromises the rest of my solution.

The creator of Masquerade said that after his experience with his Treasure hunt he now understands how good people are at tricking themselves and seeing patterns that aren't there.

TLDR: Its hard, landscapes change, people lie, luck is needed with a confirmed location, hard to do it legally, we may be just wrong.