Gods often have some nebulously-defined ability to affect the entirety of reality just by existing.
The presumed ending of the 4E Scales of War adventure path depicts the ultimate destruction of Tiamat as lessening the capacity for greed in all mortals. A specific example given is that even the temple of Bahamut becomes more charitable thanks to Tiamat's true death, going so far as to get rid of fancy religious iconography. However, it also states that another entity, such as the archdevil Mammon, may attempt to claim Tiamat's former power over greed, rise to become the new god of greed, and through godhood inspire greed in the hearts of mortals again.
That's why Orcus and other lesser evil entities desire godhood. Why would he settle for animating the undead personally when he could kill the god of death, claim dominion over death, and then change the rules of the multiverse so that every mortal who ever dies reanimates as a ghoul or something?