Why do we call the God of the universe ''Our Father''?

Because He is the creator of all things and in that sense is 'father'.

For believers who are born again of the spirit of God, God is father in another sense

Comment: Christ told us to pray to 'Our Father' in Matthew 6. He prayed to His Father in Mark 14:36. And for the curve ball, Jesus is called our Father in Isaiah 9:6.

Jesus told Nicodemus that we must be 'born again' to enter the Kingdom of God which is spiritual. No physical human is spiritual yet, therefore, those called as John 6:44 states and repent, get baptized and follow Christ are 'begotten' just like He was with God's Holy Spirit. The Spirit continues to grow in this lifetime and when called as the bride or 1st fruits of Christ at the 7th trumpet, will be changed in a twinkling of an eye or born again into the God family.