Actually they do.... or did rather
In the winter they operated MAN-CMF-NCL-CMF-MAN, Im sure there is a BFS-PRG-EDI-PRG-BFS or something very similar going on as well.
Not sure if it still goes ahead but the MAN-PRG on a sunday was ment to be a W pattern as well but not sure how it goes now.
Like Ian says, Jet2's aircraft have a bit of give in the sched's to allow for catch up with delays, so unlike easyjet if 1 aircraft goes tech you dont have to re shuffle the whole base like they do sometimes in NCL.
And as these a/c are OWNED and not leased like other airlines, whatever money they make is Jet2s and not the leasing comapany, so they dont have to fly 6-8 sectors a day