The specific issue with a drug like ketamine, unlike, say, ether, is that it is tasteless, colorless and odorless. If someone spikes your drink with ketamine, even if it isn’t alcohol, you won’t know. That is super dangerous. Much more so than plying someone with alcohol or using something like ether or chloroform that you can buy without a prescription.
I suppose you could legislate some sort of odor or taste or something to be added to ketamine, but I imagine it doesn’t have them for a reason.