Its a good point Mark, although a lot of things we do are to prolong and enhance their lives. Such as not feeding on substrate to avoid impaction and not live feeding as they can get injured too. Obviously in the wild they do both of these but as a result their lives may be shortened.
Most of us let them wonder around our homes though, and those aren't sterile... so they do come into contact with a few nasties to help build it up a bit... but it is a good point.