People go out partying on near-empty stomachs and take crazy amounts of alcoholic drinks with not so much as a drop of water all night; it's no surprise that their bodies react that extremely.
Also, sticking to drinks with low levels of congeners helps immensely. Ethanol is already toxic enough without throwing other more dangerous alcohols into the mix.
Regular beer contains about 20x as much water as it does ethanol (by volume), but apparently that's not enough. It's certainly better than wine, though, which only contains 6-8x the water and an entire catalogue of congeners.
I could see drinking water, then sobering up, and going to sleep could mean water could then be absorbed. That may limit the effects of the dehydration and therefore the hangover.
Also your body's overall health and efficiency probably pays a role. In my teens/20s I could get sloshed and wake up with merely a vague discomfort. In my 30s I started to get actual hangovers. In my 40s its now just awful almost no matter what I do.
On the plus side, I'm saving money drinking less in general...
Some is down to the metabolic products of ethanol, specifically aldehydes. There are other alcohols (like tert-Amyl alcohol) that people have used recreationally which don't cause hangovers in the same way.
There are other great reasons not to mess with these substances (tert-Amyl is very long lasting and as little as 30ml might kill you), but they do show that some of the hangover is down to the specifics of ethanol's breakdown in the body.
It's genuinely one of my big wishes! I'd pay 5x the normal price for a bottle of whisky that neither has the "good" nor "bad" parts, as if it was water but with the same taste.
However, people often tend to drink for the "good" effects that this gel prevents. If what you want is alcohol's fun, then you're going to need another cure (essentially an IV drip, and drugs to replace the hormones and loosen your blood vessels - all of which are not readily available for other good reasons).