To use an old example, a doctor does not start over washing bedpans just because he transfers to a new hospital.
Reminds me of the story of an American B727 Captain whose company went bust. Two years out of work before he got another job starting as a flight engineer on the same type, then becoming a first officer again until he could get back in the left hand seat. All up about eight years to get back to where he was previously.
The system survived due to the relatively long upgrade time in legacy airlines with ten to twenty years being normal. Nobody left and upgrades depended on someone either dying or retiring. There were plenty of ten year first officers who had been ready for promotion for the last five or six years and taking direct entry captains would have been unfair on them. DECs are acceptable where there are no first officers suitable to upgrade or for an expanding start up which needs experienced captains immediately.
Also the number of major airlines in a country is limited and can usually be counted on one hand, where as there are several hospitals in every town and city giving the opportunity to slot a doctor in at a level appropriate to his qualifications and experience without treading on anyone elses toes.