Not of their own volition so an incentive (reward) no longer applies. They have to stick around for up to 12 months to enable a suitable relief to be recruited/found, trained and assigned with all the knock-on effects (e.g. manning turbulence, unplanned extra work, inconvenience and cost) their premature departure incurs.
So why can't HM Forces cope like any civvy business? eg if I want to leave my job I give one month's notice. I'm a highly trained professional, whats the difference?