Alcohol is converted into acetaldehyde, which, IIRC is even more toxic
than alcohol, in the liver, using the enzyme alcohol dehydrogenase
(ADH).  As with most enzymes, more of it is produced when more of its
substrate is around, so drinkers have more of this enzyme in their
livers than non-drinkers, and metabolize a given dose of alcohol more
rapidly.  So they have to drink more to get as drunk as before.

Livers can take an amazing amount of abuse and continue to function,
although all that regrowth means greater likelihood of cancer and
eventually so much of the liver becomes scarred (cirrhosis) that not
enough working tissue is left to support the body.  It can take a
long time, and loss of most of the liver, before this point is
reached.  Then the person is in real trouble, while up to then they've
been able to "get away with it".

As with any psychoactive substance, people learn to function when
intoxicated, so a habitual drinker can keep going with a higher
blood alcohol level than a person who seldom or never drinks to
the point of intoxication.  No doubt the nervous system adapts to
some extent as well.

I suspect the "1 ounce per hour" rate is a rough average for most
people who don't drink heavily.  There's a lot of individual
variation, some of it genetic.  For example, people from eastern
Asia are less tolerant of alcohol, i.e. become intoxicated from
smaller amounts, than people from other parts of the world.  IIRC,
they have, in general, a lower base level of ADH, so don't process
the alcohol as rapidly as others do.