They can't if being "born again" means what you think it means.
You think it IS salvation- it is the whole of it. You base this on a viewpoint that is very new so far as church history is concerned but old enough for you to have been raised hearing it. So you cannot see being "born again" as anything other than the WHOLE of salvation.
I believe if I dig far enough (historically), that I will find that your theology is relatively new--new in the respect that salvation is not a process (not the act of salvation). I am not speaking of hearing of the Word of God, the conviction of the Holy Spirit. I am speaking of the event, when a person trusts the Lord as their Savior--that event which took place on the Day of Pentecost, or with the Ethiopian Eunuch, or the Philippian jailor, or perhaps even better, the thief on the cross.
With the thief on the cross, how long did the Holy Spirit have to work in his heart; had the Father been drawing him: was he regenerated before he was saved; etc. How long? Or did it all take place in a moment, in that very moment when he looked at Jesus and said (prayed): "Remember me when you enter into your kingdom."
Part of the trouble is that you fail to see the other side because of your environment, background, teaching, etc. I am accused of the same thing. You say things that can't possibly be true. If they were true I wouldn't be saved. That is one way I know they are false.
How long did I have to hear the Word of God before I was saved?
How long did the Holy Spirit have to work in my heart before I was saved?
How was I supposed to know that the Holy Spirit was working in my heart?
What if I didn't have a great sense of guilt or sin when I was saved?
What if my life didn't radically change right after I was saved?
What if I didn't feel any different after I was saved?
I was saved as an adult at the age of 20, with a Catholic background. I had never heard the gospel previous to the time that I was saved. The "Mass" that I went to was said in Latin. I got saved the first time I heard the gospel, because it made sense. It appealed to my intellect. I was a sinner in need of a savior, and Christ could meet that need, personally. When it could be shown out of the Bible, I had no argument. It was Christ that saves, not the Church. I prayed and trusted Christ as my Savior.
--I never sat under the preaching of the Word of God previous to that time.
I never felt the convicting power of the Holy Spirit.
I didn't feel any great sense of conviction; there were no tears, etc.
And there was no big change in my life after my salvation, at least not immediately.
I didn't doubt my salvation because it was firmly grounded in the Word of God. I didn't know any of the Word of God, but those that led me to Christ discipled me--at least three days out of the week. They made me memorize Scripture, attend Bible Studies, fellowship with other believers. I had an entirely new group of friends (trustworthy friends) that I never had before. The more I spent in the Word after I got saved (and in fellowship with others) the more my life began to change. That is a result of both discipleship and a process of sanctification.
I can give you the exact date--right to the hour--of my salvation. There is no reason for me to assume there was any process going on before that time, for there wasn't. I wasn't raised in a Christian family.
The event of salvation includes regeneration, justification, adoption, sealing of the Holy Spirit, initial sanctification, forgiveness of sins, redemption, and so much more. God gives us all these things at once.
He that spared not his own Son, but delivered him up for us all, how shall he not with him also freely give us all things? (Romans 8:32)