[annalex]

Because it is the will of God.

The will of God is that we use our own will. Then, and only then He will work in us. "By works a man is justified; and not by faith only".

[Dr. Eckleburg]

James was speaking to believers, to exhort them to put their faith into practice.

"Yea, a man may say, Thou hast faith, and I have works: shew me thy faith without thy works, and I will shew thee my faith by my works.

Thou believest that there is one God; thou doest well: the devils also believe, and tremble." -- James 2:18-19

Works are the fruit of the Holy Spirit and by them we are known.

DOES JAMES CONTRADICT PAUL?
by John Piper

And...

OF GOOD WORKS
by A.A. Hodge

And...

THE ROMAN CATHOLIC TEACHING ON
SALVATION AND JUSTIFICATION
BY William Webster