For two reasons. First, in view of His Son Christ, who is the eternal, essential wisdom of the Father, who was begotten of Him from eternity (Proverbs 8:23), and after taking upon Himself a human nature, was revealed as the Son of God (Romans 1:4; Luke 1:35).
3 concerning His Son, who was born of a descendant of David according to the flesh, 4 who was declared the Son of God with power by the resurrection from the dead, according to the Spirit of holiness, Jesus Christ our Lord, Romans 1:3,4
34 Mary said to the angel, “How can this be, since I am a virgin?” 35 The angel answered and said to her, “The Holy Spirit will come upon you, and the power of the Most High will overshadow you; and for that reason the holy Child shall be called the Son of God. Luke1:34,35
Second, because He promised and actually demonstrated that He wanted to be our Father too when He gave us His natural Son to be our brother, incorporated us by faith into His Son as true members, and thus accepted us as His children.
In John 20:17 Christ says, “Go to my brethren and say to them, I am ascending to My Father and your Father, and to My God and your God.” Thus we are taught by the name “Father” that in Christ a tight bond between God and us has been established and that His love toward us, His children, is unwavering.
In John 17:11 Jesus says, “Holy Father, keep through your name those whom you have given me, that they may be one as we are.” And (in John 17:23), “that the world may know that I was sent by You, and that You have loved them as You have loved Me.”