Ethanol is metabolised in the primarily liver into acetaldehyde which slowly damages the liver, if you drink enough to process more your live can handle it will damage the live. It also leads to fatty liver disease which cause metabolic problems. Being fit and a healthy weight doesn't make much difference.

It's the same reason why a diet high in sugar cause the same problems; as sugar is 50% fructose which is metabolised by the live in the same way as ethanol.

A pint of Heineken (very popular lager in the UK) is 225 calories. For a small person, 6 pints might be the equivalent of their daily food intake. Anecdotally speaking, drinking doesn' fill me, and I end up either snacking on nuts (which are also super high calorie), eating chips, or just eating a meal anyway.

Of course you can drink heavily and not have weight problems, in the same way you can eat cake and frozen pizza exclusively and not have weight problems, but you would be lying to say they don't make you fat reeeeeal quick.