insufficiency of material proof for hearts hardened in disbelief

وَلَوْ نَزَّلْنَا عَلَيْكَ كِتَـٰبًا فِى قِرْطَاسٍ فَلَمَسُوهُ بِأَيْدِيهِمْ لَقَالَ ٱلَّذِينَ كَفَرُوٓا۟ إِنْ هَـٰذَآ إِلَّا سِحْرٌ مُّبِينٌ

And even if We had sent down to you, [O Muḥammad], a written scripture on a page and they touched it with their hands, the disbelievers would say, "This is not but obvious magic."