why dont adults come to the after life?

I thought they were the teachers...

That version of the afterlife is for young people who never got to do what they really wanted to in their lives.

There are adults. Did u watch the part when yuri's group went underground to take some weapons they made?

Adults do come to the afterlife. The conditions for people to enter that world are...

They must have led a horrible life and died.

Happened while they are still being of in the age of education. (So, going to school)

Age does not matter.

Like Chaa, the one in charge of the Guild. He's actually an adult, but his time of death was around the time when he was going to highschool.
(read the manga)

I'm pretty sure where angel beats takes place is only a part of the after life. To be their u have to be a high school student who had an unfufilling life. I'm sure adults go somewhere else if they had an unfulfilling life.

There were some adults but i think it's a purgatory mostly for teens and young people. maybe the adults have their own afterlife?