LF: Just a reminder that what you state is for Nusach Sfard. I daven Nussach Ashkenaz so the last tefillah is the “Shir Shel Yom” not Aleinu.
Why do people leave early? For some it is car pool issues. Their sons have to be in Yeshiva at a certain time and the school does not provide bus service. The fathers have to drive them to yeshiva. For others it is their work schedule. They may not have a minyan near where they work so they daven first with a minyan and then leave to go to work.