Only Pilots carry out the Preflight checks in this Airline so as to save on costs.In my 50years as a Licensed Aircraft Engineer in several parts of the World including 22years in Australia.In my experience Pilots do not have the expertise to find defects of any significance especially in the 10-15minutes on Preflights and I would not expect them to either.This is why we have these incidence as you have reported.
Tadick.