simple, because they can make money out of your misfortune.

lost license fee £20 kerrching,
removing points £20 kerrching

etc etc etc

I actually believe they throw a certain amount of recieved mail into the bin as a "local" management policy when workloads are high, just like the postie "you were out" scam for parcels to save time. when i sent letters for my medical issues to and from DVLA of 6 letters sent only 3 arrived. Now royal mail are bad but they aint that bad.
DVLA have even lost signed for stuff.