Searching is a surprisingly subtle activity. As AKSAR put it,you are looking for clues,not just human size objects, and these are often not very obvious. A competent searcher must be focused and aware. It doesn't help if you are dehydrated, sore, distracted, etc. You will miss important stuff. This has been demonstrated way too many times.

when I am searching (or rather, when I was searching), I could always tell when I was alert and focused by the fact that I was also noting archaeological material. Archaeology is my profession and I have done a lot of archaeological survey - looking for traces of human activity in the landscape. This is an activity that is obviously closely related to SAR, and the same constraints apply. It is simply amazing what people can miss if they are distracted, disoriented, or confused.

Searching and SAR work are like most human activities. You get better with practice and experience, especially if guided by decent training. If your garden needed weeding, would you crowd source that, or turn to an experienced gardener?