Why people leave?

From the beginning of this year, serverl engineers in my department leave.
I think this is bad situation. But I donot know why they leave.

I know salary is low in our company and the work is very busy in last year.
But the project is still not finished, why do they consider the company's benefits?

Actually, our project had been affected by those people who leave. This make me very
upset.

Maybe different people have different aim, and young people have different sense of worth with old people like me. It is difficult to tell who is right or wrong.