My understanding is that dead drunk is someone who is SO drunk he is absolutely in a stupor – you could not wake him with a foghorn in his ear, or a bomb lit under him. That is someone who is passed out cold, and will likely wake up with the grandmother of all hangovers.
Most people do not get dead drunk, merely plastered. They still act stupidly, and what is worse, because they are drunk they have no idea exactly how stupid they appear. Ever see a belligerent drunk? That’s the worst. And it can happen after only a very few drinks. It depends on the nature of the person drinking. Isn’t there a saying that a person’s true nature only comes out when he is intoxicated?