I agree that in Romans 4, why people believe is not addressed.
But in other parts of God's word, it is.
For example, Philippians 1 says that belief is a gift:
" Only let your conversation be as it becometh the gospel of Christ: that whether I come and see you, or else be absent, I may hear of your affairs, that ye stand fast in one spirit, with one mind striving together for the faith of the gospel;
28 and in nothing terrified by your adversaries: which is to them an evident token of perdition, but to you of salvation, and that of God.
29 For unto you it is given in the behalf of Christ, not only to believe on him, but also to suffer for his sake;
30 having the same conflict which ye saw in me, and now hear [to be] in me."
In the above, Paul tells the Philippians ( in verse 29 ) that it is given to the believer both to believe, and to suffer for His sake.
So, anything that is given by God is a gift;
An example of this would be the ultimate gift, eternal life...
which is to know God and His Son ( John 17:3 ):
" For the wages of sin [is] death; but the gift of God [is] eternal life through Jesus Christ our Lord." ( Romans 6:23 ).