I guess the only thing I have to add to what others said is that it's important to keep in mind historical context, about when and why these ideas became canonized.

Existentialism isn't my discipline, but it seems to me most of these dudes were either a. directly addressing the death of "divine right" in western society (existence is determined by our choices and actions, not pre-determined by god's command) or b. addressing the inhumanity of the world wars (we must remember that the atrocities of these wars were determined by the actions of men, and not some mystical force that cannot be understood or adressed.)

Which.... yeah is unhelpful now but that wasn't always the case.