Why does beer last so long?

Strange thoughts came over me as I was 'plodding' around the M42 yesterday. Why does beer last so long? When I was a teenager I could down 6 pints before you could say cheers....... Now you buy a six pack of small bottles and after around 1.8 litres (just over 3 pints) you think enough. Don't feel the slightest bit drunk but my body is saying oiii stop it!

So you buy an 8 pack and it lasts for more than one day.