In the past, according to stories from people that defeated Behemoths, was that they carry a lot of magic stones in their stomachs.
When people heard that, they made plans on getting rich quick.
But taking down a Behemoth was difficult.
The skin was extremely firm and hard, with a tough body that doesn't flinch when facing ordinary attacks.
Although there are ways to attack, just having that giant body rampaging is enough to be a threat.