There are probably not as many missing as you think. One of the problems is that cache owners can’t be bothered to mark them as missing when clearly they haven’t been in the cache for months. If I notice that a trackable has been missing for several months I always add a comment asking the CO to mark it as missing, then it won’t show up in the cache.
Obviously some do get stolen, but probably not as many as it appears. I normally only place trackables in remote caches as I think they are safer there than urban caches or C&Ds.
NEVER place trackables in National Trust caches as they loan out GPSs to complete beginners who would not know what a trackable is and may treat them as swaps 🙁