A quick Google suggests the three major passenger airline alliances did a combined total estimated 50,000 flights per day in 2014, that's 18,250,000 fights in 2014 !
So the POTENTIALLY "untraceable" loss of 2 or 3 aircraft, whilst obviously sad, is a mere needle in a haystack at 0.00001643835616438356%. And that's just the number for the "alliances", if you consider the other non-alliance airlines, that percentage becomes even smaller !
It does not matter in this case how many flights a given plane makes. What matters to insurance is that X airframes out of Y insured are gone. The number of actual planes is a percentage of flights a day, as most are in the air daily and several times. According to Boeing ther were 20000 commercial aircraft in the world in 2012. 3 of those are 0.015%. Perhaps that seems small, but it sure has a lot fewer zeroes past the decimal point.