You wouldn’t, but Scripture does – it says the Cross redeemed all mankind , not just believers. For example:
“he is the expiation of our sins, and not for ours only , but the sins of the whole world ” (1John 2:2);
“Then as one man’s trespass led to condemnation for all men , so one man’s act of righteousness leads to acquittal and life for all men ” (Romans 5:18);
“He entered once for all into the holy places, not by means of the blood of goats and calves, but by means of his own blood, thus securing an eternal redemption” (Hebrews 9:12);
“Behold, the Lamb of God, who takes away the sins of the world ” (John 1:29);
“But God shows his love for us, that while we were still sinners , he died for us” (Romans 5:8).
So Jesus died for all mankind – past, present, future – which includes not just believers, but unbelievers as well. We did nothing to be redeemed by the Cross – it was a totally unconditional gift from God - so even unbelievers and haters of God are redeemed.
This means, of course, that redemption and salvation are not the same thing.
In the context of Christianity, to be “saved” means to be saved from eternity in Hell, thus spending eternity in Heaven instead. So it is incorrect to say “all are ‘saved’ by the cross”, because that would mean:
(a) everyone who has ever lived or ever will live will gain eternal life in Heaven (even those who hate God), and
(b) gaining eternal life in Heaven would not require faith in Christ, because the Cross has already “saved” them.
The Cross per se does not save; it redeems – the Cross makes it possible to be saved. So all mankind is not “saved” by the Cross; rather, all mankind is redeemed by the Cross.