Why?
Short answer: Because they can.
Long answer: Because at college age, in most countries people are over or near legal drinking age, are more independent from their parents, so they *can* drink.
What happends after college life is serious adult life. Meaning having to be responsible *sigh*, holding a steady job *sigh* or starting a family *sigh* or other boring things where a lot of drinking isn't very compatible with.
When going to college or university, people have both a lot more freedom to party, and it's probably the only time period where they actualy can party that much.
Couple that with a forbidden fruit alchohol culture, all the "hip cats" doing it, and at college age still being sensitive to peer pressure, ét voila.
Have you noticed how college binge drinking is considered *normal* or even mandatory, while anyone over 30 getting drunk regularly is either called a loser or an alchoholic? 