-Avraham, Yitzchak and Yaakov (Abraham, Isaac, & Jacob) were all shepherds, so were many other great leaders. (Moshe, David)
-All these people had money to hire workers.
-They chose to do so they could meditate.
-This would allow them to stay out for days at a time.
-Air in the fields was clear.
-Praying and meditating among grass and trees works better then in the city
-All the angels appointed on the grass come and join the prayers
(Medrish Talpiyot branch of Avos)