The Beale Cipher is indeed one of the modern mysteries of cryptography, and the fact that it is mostly unbroken can either mean one of two things: the answer continues to allude the thousands of cryptanalysts and treasure hunters who have tried thus far, or the cipher is indeed faked, and there is no actual solution. If it is indeed the latter and the cipher is a fake, then there is no reason for people to continue. There would be no treasure for the treasure hunters, and there would be no actual solution to find for the people who just want to solve the cipher.
However, the fact is that no one knows for sure whether the cipher is a fake or not, meaning that there is a chance that there is an actual solution, and maybe even an actual treasure.
It is unlikely, however, that at this point in time the actual worth of the treasure has any significant effect on the desire to crack the code, but it is possible. The fact that so many people have failed to crack it means that no one person can really expect to solve it, hence the likelihood they solve the code and find the treasure must be really small. Therefore, the majority of the motivation for looking for the solution is probably just the mystery that surrounds the code, the fact that it is unsolved. These are the types of people who are doing it for fun, because they can. What better reason is there than that.
Image Credit: "Beale Papers" Wikimedia Commons (Public Domain)