Maybe a reason we have crashed UFOs isn’t because us humans have shot them down. I mean the logic that a craft is advanced enough to travel all the way to earth just to crash is pretty illogical. Humans shooting it down is only slightly more logical. But what if we have other UFOs out there shooting down unwanted other UFOs? Maybe the zoo hypothesis is correct and these crashed UFOs are poachers in a sense. Or some other interaction between different types of UFOs/Aliens.