To be alive, you basically need two things: a living spirit/mind, and a fully functioning body. Note that clothing is not included in there. When people are born as babies, they're naturally Naked on Arrival, and the same often goes for resurrections, unless the force that brings people back to life is feeling extra generous to give them a spare set of clothing on top of that. This can be symbolic of their rebirth into the living world, of course, or serve to show how fragile life is by having them be vulnerable upon being thrust back into the world. Fanservice is also a contributing factor.