Why does Satan tempt us? I mean, I know why he does it, but why is he able to? Why doesn’t God make him stop?
Fr. Jose Antonio Fortea gives the following explanation:
Why does God allow temptation?
If God does not want us to sin, why does He allow temptation at all? We have the answer in the verse that says: Count it all joy, brethren, when you meet various trials, for you know that the testing of your faith produces steadfastness (James 1:2)
Without temptation, there would be no opportunity for us to grow in virtue every time we successfully resist the allure of sin and remain faithful to God’s will. It is true that God could restrain the demons so that they would be unable to interfere in the history of humanity, but He allows their diabolical work so that it can be an occasion of virtue for us. In other words, we could say that God permits a certain amount of darkness in this world (i.e., temptation and sin) so that a purer and brighter light (i.e., virtue and salvation) can be gained. So God allows temptation because He knows that much good can come from it if we resist.
Source: Interview with an Exorcist: An Insider’s Look at the Devil, Demonic Possession, and the Path to Deliverance pp. 51-52