How did infantry front lines were convinced in ancient wars to be put in a position that pretty much means certain death?

As far as I know, front lines in wars were almost always decimated

Eh, not quite. It's riskier up front, but there are other considerations. More can be said on the matter if anyone else would like to address the matter of who's up front; for the meantime, here are some previous posts on this:

u/Iphikrates and u/Iguana_on_a_stick on how this applied for the Greeks and Romans;
while u/Hergrim examines both how it went for the Medievals and how casualties actually accrue, and in a previous post also linked there, examines the willingness to get into close combat.