These scenes can be only seen on the Last Day. Allah (swt) makes what we actually see in life a proof about the truthfulness of what we have not seen of the hereafter matters. Since this speech is about the future, (will) is used.

This is Allah’s wisdom, that is to set forward the perceptual sensible matter to as a proof of the followed unseen one. Hence, the two first verses refer to issues in life while this third one is an indication about the hereafter.