Because belief of God's words is a result of the new birth...
The new birth does not depend on our belief of His words.
John 1:13 says that the new birth was not a result of the three things being described...
Bloodline, the will of the flesh and the will of men.
James 1:18 says that God "begat" us with His word by His own will.
I take that to mean that the new birth ( God's "begetting us" ) was not of our will, because John 1:13 tells me that it was not.
Therefore...
Hold up there.
John 1:12
But as many as received him, to them gave he power to become the sons of God,
even to them that believe on his name: 13
Which were born, not of blood, nor of the will of the flesh, nor of the will of man, but
of God. KJV
I believe this being born of God is to defer from Judaism or any other religious means to become the sons of God by, now that Christ has come, died, resurrected, & ascended at the right hand of God the Father in Heaven so by simply believing in Him is how we are saved and our believing in Him is a work of God..
James 1:17
Every good gift and every perfect gift is from above, and cometh down from the Father of lights, with whom is no variableness, neither shadow of turning. 18
Of his own will begat he us with the word of truth, that we should be a kind of firstfruits of his creatures. KJV
Confirms that our believing in Him is from God, not from our flesh nor will power. Our believing in Him is a work of God.
The saved believe because they are born again, the unsaved never believe / tares fall away during trials , because they were never given the new birth and were never indwelt by the Holy Spirit after their "belief".
The parable of the seed & the sower is referring to unbelievers that were not able to receive the seed, even though hearing it, crows as in unbelief gobbled it up so the seed was not received.
The others that did receive the seed had received salvation but how they grow in the Lord will be judged as to how fruitful they will be as His disciples.
Lack of the new birth = Automatic rejection of God's words as being foolish or not to be regarded as taken seriously.
Presence of the new birth / regeneration = Being enabled to believe the words of God.
Hence the crows gobbled up the seed so it was not received when heard it not mixed with faith aka belief to receive that seed..
He that is "of God" ( elect ) are the only ones who will truly ( from the heart ) believe when they are preached His words to...
He that is not "of God" ( lost, reprobate ) will not believe / cease from believing or continuing in His words and in their hope in Christ.
Clarity & reproof in regards to how you are applying that parable to mean. Reprobate means the disqualified from attending the Marriage Supper in Heaven in regards to the race we are to run by faith in Jesus Christ for the high prize of our calling to be received by Him as that vessel unto honor in His House as the crowns we may receive are actually His crowning achievements in us for why the elders were casting their crowns & our crowns at His feet in Heaven.
Jesus will go after that lost sheep per Matthew 18:10-14 because all lost sheep have been bought with a price & sealed as His Ephesians 4:30 for why we are not to grieve the Holy Spirit nor defile the Temple of God or else per 1 Corinthians 3:10-17 Those unrepentant saints & former believers left behind, He will get them as they will be resurrected along with new believers at the end of the great tribulation. 2 Timothy 2:10-26 explains why we seek the good of saved believers in discipleship.
Stated another way, I see that Romans 1:18-32, Romans 3:10-18, and several others keep us, as a race, from believing God's word.
Romans 8:29-30 designates who will believe them.
Romans 9:13-24 tells me who believes and who does not and why.
Romans 10 tells me who believes and how ( by the preaching of His word ) and also that many reject it, by using the example of Israel as a nation.
I hope that helps to explain things.
Discipleship aka running the race is not for salvation because we cannot run that race without Him in us & helping us lay aside every weight & sin. We run that race as saved, but for the high prize of our calling or else risk not only losing our rewards of crowns ( His crowning achievements in us ) but in becoming a castaway too as Paul warned even himself about.
Hebrews 12:1Wherefore seeing we also are compassed about with so great a cloud of witnesses, let us lay aside every weight, and the sin which doth so easily beset us, and let us run with patience the race that is set before us, 2 Looking unto Jesus the author and finisher of our faith; who for the joy that was set before him endured the cross, despising the shame, and is set down at the right hand of the throne of God. ~ KJV
1 Corinthians 9:24 Know ye not that they which run in a race run all, but one receiveth the prize? So run, that ye may obtain. 25 And every man that striveth for the mastery is temperate in all things. Now they do it to obtain a corruptible crown; but we an incorruptible. 26 I therefore so run, not as uncertainly; so fight I, not as one that beateth the air: 27 But I keep under my body, and bring it into subjection: lest that by any means, when I have preached to others, I myself should be a castaway. KJV
Those left behind may lose their crowns & be resurrected after the great tribulation as vessels unto dishonor in His House, but they are still saved regardless. It is liken to the prodigal son losing his first inheritance but find he is still son.
The works on that foundation gets burned up but the foundation remains. Their spirit is saved as that seal of adoption is not going anywhere from the spirits of the saints for when they are absent from the body during the great tribulation, they shall be present with the Lord.
2 Corinthians 5:7 (For we walk by faith, not by sight

8 We are confident, I say, and willing rather to be absent from the body, and to be present with the Lord. KJV
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