There are a lot of reasons of kufr (disbelief) and does not always depend on miracles. Likewise, there are a lot of reasons for belief, not always involving miracles. One of the main reasons is al-fitra (natural disposition).
فِطْرَةَ اللَّهِ الَّتِي فَطَرَ النَّاسَ عَلَيْهَا
Today we have 1.5 billion Muslims that didn't see miracles that had been seen by sahaba and quraich people, and they are believers. We have thousands of new Muslims every year, which didn't know a lot about the Quran. And they don't know Arabic like quraich the people.
And you said that there are even some Christian Arabs who believe that the Quran is a miracle, yet they don't believe in Islam. That means there are reasons that make them reject Islam.
Why do Arabs who have received a very clear message reject it?
As said above, there are a lot of reasons people reject these messages, some of this are in Islamic stories and ahadith and narrations, and some others in the Quran, and some in both. To avoid translation narrations and find them, I will count some reasons that had been mentioned in Quran.
Ingratitude and obstinacy: Denial the truth even after know it.
وَجَحَدُوا بِهَا وَاسْتَيْقَنَتْهَا أَنْفُسُهُمْ ظُلْمًا وَعُلُوًّا
And they rejected them, while their [inner] selves were convinced
thereof, out of injustice and haughtiness
Pride and arrogance: and the example of Satan is very clear.
وَإِذْ قُلْنَا لِلْمَلائِكَةِ اسْجُدُوا لِآدَمَ فَسَجَدُوا إِلاَّ
إِبْلِيسَ أَبَى وَاسْتَكْبَرَ وَكَانَ مِنَ الكَافِرِينَ
And [mention] when We said to the angels, "Prostrate before Adam"; so
they prostrated, except for Iblees. He refused and was arrogant and
became of the disbelievers
Intolerance towards parents and grandparents:
وَإِذَا قِيلَ لَهُمُ اتَّبعُوا مَا أَنزَلَ اللّهُ قَالُواْ بَلْ
نَتَّبِعُ مَا أَلْفَيْنَا عَلَيْهِ آبَاءنَا أَوَلَوْ كَانَ آبَاؤُهُمْ
لاَ يَعْقِلُونَ شَيْئاً وَلاَ يَهْتَدُونَ
when it is said to them, "Follow what Allah has revealed," they say,
"Rather, we will follow that which we found our fathers doing." Even
though their fathers understood nothing, nor were they guided?
Envy and jealousy:
وَدَّ كَثِيرٌ مِنْ أَهْلِ الْكِتَابِ لَوْ يَرُدُّونَكُمْ مِنْ بَعْدِ
إِيمَانِكُمْ كُفَّاراً حَسَداً مِنْ عِنْدِ أَنْفُسِهِمْ مِنْ بَعْدِ
مَا تَبَيَّنَ لَهُمُ الْحَقُّ
Many of the People of the Scripture wish they could turn you back to
disbelief after you have believed, out of envy from themselves [even]
after the truth has become clear to them
Prophet accused of being a magician:
وَإِن يَرَوْا آيَةً يُعْرِضُوا وَيَقُولُوا سِحْرٌ مُّسْتَمِرٌّ
And if they see a miracle, they turn away and say, "Passing magic."