Since it appears that no actions by the Serbs resulted in anything close to those inflated numbers actually dying, why should they accept responsibility for them?
For the same reason why neighborhood bullies might try to force their victims to assert some absurdities. It is to show their power and to intimidate others.
There is other factor - after Serbs (or their leaders) accept this "responsibility" it will establish the ground for future and perpetual reparations (a good source of income for the pet projects). Also it will seal the conviction in official image in the mind of general public ("Serbs admit it themselves so it must be true")