Question

Why open field and the tall trees are not safe during thunderstorm

Answer 1

As one must understand lightning is a discharge of a huge of electric charge. So, I hope this helps you to get an idea about what exactly happens when a person is present on an open field.(he/she might get electrocuted)
And why not under tall trees!? --because the discharge of electric charge prefers conductors of narrow cross section, here a single tall tree behaves as the same.
I hope this helps you.