Magic radiates off of objects and areas. Detect magic can be thought of in the same was as a passiver radar system. The passive radar system doesnt transmit, it only recieves.
Whereas evil/good/undead/chaos/etc are at the core of a creatures "soul" (for lack of a better word). It doesn't radiate per se. I know clerics have an appropriate aura, but aura and radiate are not the same thing for this example. So you would have to use a more difficult active system to locate the evil/undead/good/etc. The spell sends out "radar" and recieves reading. Its the magical equivelant of a real life active radar system.
In short passive is easier and active is harder. Hence the level difference.
Also, a something being Magic doesnt necessarily mean you should kill it. However if it's evil/undead most characters (being good aligned) would feel the urge to hit bash it in the face with sticks. So basically it's a minor game balance issue.
Thats the best rationale I can think of.
Detect magic used to be a first level spell before Cantrips were part of the official rules. Now that the rules have changed to have 0-level spells it comes down to logic about the use of the spell. Detect evil and detect undead are not necessary spells to function in the world of magic, however how can you explain a spell caster who is a master at working with spells not begin able to detect magic. Making detect magic a 0-level allows all spell caster access to it without yanking one of their limited first level slots to know and use it.
My meta answer is while both have strategic uses, detect evil and undead are primarily tactical (location and disposition of the enemy to help in the fight), while detect magic is primarily bookeeping (seeing what of the loot is what, after the fight). As a design choice, resource management is more heavily weighted on the tactical side than the bookeeping side, so (occasional magic detectable enemies notwithstanding) IDing the enemy costs precious slots, while IDing loot is free.
I think the main reason is the 'required' detect magic spell after a battle to check for loot quickly and without fuzz. I'd rather have seen casters being able to swap out any spell of 1st level and up for a detect magic at any time and make a 0-level version that functions on touch maybe.
I just think they'd done it different if they had given it some more thought.
I play intrigue and mystery heavy campaigns and Detect Evil at will, or any other alignment spell at will, would totally screw with adventure design as well as global politics. Detect Undead likewise has effects for sentient creatures such as vampires or liches who may be trying to pass themselves off as living. Doing a cursory scan of the party reveals someone is undead, focusing makes you obvious but narrows the field, and go from there.
So yea, having unrestricted access to some stuff like that makes it more difficult to build & manage the world as a DM.
So yea, having unrestricted access to some stuff like that makes it more difficult to build & manage the world as a DM.
Reminds me of the 4e eladrin; having an entire race whose commoners can teleport regularly makes it hard to build a sensical world. There's a few things that I never allow simply to build a coherent world, teleportation and planar travel are the big ones. Also not making certain magic items not available can really help.
The thing about detect undead that makes it useful every once in a while is the fact that both magic and evil have spells that can mask them, but I'm not aware of a spell that masks an undead from being detectable as undead.
Nondetection and misdirection to name a few. Most successful necromancers would research other masking spells as well, but it is matter of specific campaign instead of generic.