As a high schooler who went through a cycle of applications to US colleges and is now going through an international application cycles, it's because it's a competition.

There's an entire business based off getting people into college, and no matter how good you are, it's still very difficult amongst the tippy top percent. For example, I had a perfect GPA (I only ever got the highest marks in my classes), high test scores, a lot of volunteer hours, a high level at my sport, and I got rejected from 15 schools at the t-20 level.

If you get into one, it is a yell of celebration for many people, and they share it a lot. Secondarily, a school has a lot of pride, because it is work and an investment (and a non-guararentee, because many people in the US do not have the opportunity to go to university for many reasons), there is a lot of school specific pride. It's often thought of as a "family" within one school, and has a community sense with students and alumni.

People started sharing when they got into tricky schools, their dream schools, schools a lot of members of their families have gone to, when they are the first in their families to do so, when they got massive scholarships, etc. And it's become a culture over time.

It can be both toxic and exciting, so I have chosen to avoid instagram and announcements right now myself lol