Because James is telling us that faith, alone, is insufficient. It can be said that Lucifer and all his minions: fallen angels, demons, et al, believe that "Jesus is who He says He is The Son of God/The Messiah, the resurrection and the life" and that this, in their hate and pride and obstinate rebellion, only makes them hate Him more. James was opposing a mentality that says, in a way, that we, too, can stay as we are, that believes that the sheer act of faith in certain truths equates to righteousness, and therefore salvation, causing confusion about whether or not they must also be changed in thought, word, and deed for the better, with many apparently becoming antinomian.
But man is to be clothed in a far more glorious robe than faith when he meets God; God will recognize His own by the love that radiates from them, reflecting His own love. The gifts of faith, hope, and love are supernatural gifts of grace, but they must be acted on, expressed, and grown: fruit-bearing. The leaders in John 12:42-43 believed but they would not profess because,
"they loved human praise more than praise from God".
I don't think that's a real good sign regarding their salvation. Love, especially, as the most important of the virtues, acts for the good of God's kingdom and for neighbor, spreading the good news of His goodness, mercy, love, and promise and desire for all to be with Him in sublime, uncompromised happiness while opposing injustice and addressing the needs of the poor, downtrodden, sick, imprisoned, depressed, addicted, victimized, sinful, hopeless in a world still far removed from that blessed state with Him.