Hi Cees,
I think one of the reasons wreckage is still to be found is that many of the locations are pretty remote. Crash sites which are/were easily accessible were cleared up pretty efficiently by the RAF MU's (Maintenance Units) in the months/years afterwards.
In more remote locations the MU's concentrated on removing the more 'sensitive' equipment and parts and the rest was simply left behind (it was before the days of helicopters). Heavy stuff was removed by using hill ponies if it was not possible to get vehicle access.
In more recent years some wreckage has been recovered from more remote locations by helicopter for one reason or another.
A Wellington crash site I visited at the weekend which was at around 1000 m was cleared in 1986/1987 although there is still plenty of evidence left today the big stuff like the engines, turrets, wings and fuselage have gone. I slept out on the hill hoping to visit an Oxford crash site the following day but woke up in the morning to a blizzard so had to abandon that plan! And this is supposed to be summer? 
Regards
Linzee