This is an old issue which was addressed by Dragon Magazine/TSR staff, as I vaguely recall, back in the 1980's-1990's. I could be entirely off, but I believe their answer was that undead are unnatural, and Druids are nature (natural) priests, who have no control over the un-natural things of undead. Basically, undead are so totally outside of their powers, that they cannot affect them, at all, except with damage-dealing spells.
If someone can correct me, please do.
For my campaign, I run strictly Specialty Priests (I abhor the "Generic Cleric" concept from 1e rules...). For my god of death, one of the Granted Powers of his clerics, is that they Turn Undead at two levels higher than they are; they also cannot control the Undead, they can only destroy them, as they are an abomination. He, like the Anubis example above, is livid with Undead escaping the Underworld (known as, Gram Nar, which is Icelandic for, Angry Corpse). If he had his way, each and every mortal soul would be contained, forever, in the Underworld, with none going to the new version of heaven (long, complicated mythology).
He is decidedly evil, and angry at the other gods, who stuck him in his position as gatekeeper of Gram Nar. His Clerics also use fear, intimidation, and a healthy, ongoing scheme of coercion to bring Humans into line, worshiping him, as they rightfully should, blah, blah, blah...
Designing Specialty Priests for each god in your pantheon is a complicated, challenging, blast! It really makes the Cleric class come to exciting life within your individual campaign world. It creates incredible role playing opportunities, as well. My mythology is so full of strife, conflicts, and open hatred, among the gods, their temples, and their Priests. Bread and butter for my players to enjoy. My game world's mythology permeates every aspect of the PC's lives. It is so intertwined in nearly everything they do.
If you examine historical European medieval history, you will see how religion permeated every aspect of their society. You will further see how much it influenced their very existence. That is what I try to achieve within my game world. My players love it, with several of them running Specialty Priests, of different gods. It is challenging for them, and for me. It makes for fantastic games, stories, and memories. It really helps make the game world come alive for all of us. My players are heavily invested in the religious-political intrigues of my game world, that they come back hungrily, to each session to dig into the issues, to thwart their enemies, to change what has been wrought by evil Priests, to establish things the way they ought to be.
My one son tells me that I've ruined him for playing in other campaigns with other GM's. They don't typically use political intrigues, let alone religious intrigues. In other games he has played in, people brag about their character sheets, and what wonderful toys they possess. He shares epic tales of intrigue, how he and his party thwarted coups, won wars by recruiting allies, organizing armies, convinced Kings to join them in their causes, etc. He understands the concept of "the power of flesh", over the "power of steel", as portrayed in the first Schwarzenegger Conan movie, with Thulsa Doom. He pursues the "power of flesh", over grabbing magic items. His PC was framed for an attempted assassination of his Emperor, and because of his connections with numerous Kings, and nations, six countries were willing to go to war, to avenge his wrongful execution, if it occurred. I call that true power -- greater than any artifact, or relic a PC might possess. That whole mess came about, due to religion, within the game. Cheers!
