The way the monkey's paw got its magic remains a mystery to the reader since Jacobs does not explain this at all. We can only look at the result of possessing the talisman and take any lessons of life from that!
Remember that the characters and events within any work of fiction are not real at all. Nothing outside the writer's own explanation exists whatsoever. A reader may speculate on an eventual reason, motive, outcome, etc, but this nevertheless remains within the realm of conjecture and not fact.
Sometimes writers relate a story in a simplistic way just to keep a story line going and to not be encombered by details. It's an easy way 'to tie everything up' in a neat little package. In this particular case, however, it seems as if Jacobs is intentionally enigmatic about the paw's origin to enhance the tone of mystery in the tale. And it works, doesn't it!
Check out the reference below concerning the relationship between British colonalization and one of the themes of this story. Interesting insight into an age-old problem....
The monkey's paw, an innocuous little paw which has dried up into a mummy, purportedly received its powers through magic. Sergeant-Major Morris, who got the paw on a visit to India, claims that "an old fakir...a very holy man...wanted to show that fate ruled people's lives, and that those who interfered with it did so to their sorrow". For this purpose, the fakir put a spell on the monkey's paw, so that "three separate men could have three wishes from it". It is intimated that by taking advantage of these wishes, those three men would learn, to their sorrow, that it is not for man to tamper with fate.
The Sergeant reveals that the first man who made three wishes was so devastated by the result that he at last asked for death. The Sergeant himself has taken advantage of the second three wishes, and his ghastly demeanor in relating this fact indicates that his wishes did not turn out well either. At the suggestion of his son Herbert, Mr. White, who has secured the monkey's paw from the Sergeant, wishes for two-hundred pounds, just enough to pay off the loan on his house. As predicted, his attempt to alter fate ends in tragedy, when the Whites receive exactly two-hundred pounds the next day in compensation for their son Herbert's accidental death.