Why didn't the Qur'an show a physical miracle?

وَأَقْسَمُوا۟ بِٱللَّهِ جَهْدَ أَيْمَـٰنِهِمْ لَئِن جَآءَتْهُمْ ءَايَةٌ لَّيُؤْمِنُنَّ بِهَا ۚ قُلْ إِنَّمَا ٱلْـَٔايَـٰتُ عِندَ ٱللَّهِ ۖ وَمَا يُشْعِرُكُمْ أَنَّهَآ إِذَا جَآءَتْ لَا يُؤْمِنُونَ

And they swear a solemn oath by Allah that if there come unto them a portent they will believe therein. Say; Portents are with Allah and (so is) that which telleth you that if such came unto them they would not believe.

وَقَالُوا۟ لَن نُّؤْمِنَ لَكَ حَتَّىٰ تَفْجُرَ لَنَا مِنَ ٱلْأَرْضِ يَنۢبُوعًا

And they say: We will not put faith in thee till thou cause a spring to gush forth from the earth for us;

وَقَالُوا۟ لَوْلَآ أُنزِلَ عَلَيْهِ ءَايَـٰتٌ مِّن رَّبِّهِۦ ۖ قُلْ إِنَّمَا ٱلْـَٔايَـٰتُ عِندَ ٱللَّهِ وَإِنَّمَآ أَنَا۠ نَذِيرٌ مُّبِينٌ

And they say: Why are not portents sent down upon him from his Lord? Say: Portents are with Allah only, and I am but a plain warner.

أَوَلَمْ يَكْفِهِمْ أَنَّآ أَنزَلْنَا عَلَيْكَ ٱلْكِتَـٰبَ يُتْلَىٰ عَلَيْهِمْ ۚ إِنَّ فِى ذَٰلِكَ لَرَحْمَةً وَذِكْرَىٰ لِقَوْمٍ يُؤْمِنُونَ

Is it not enough for them that We have sent down unto thee the Scripture which is read unto them? Lo! herein verily is mercy, and a reminder for folk who believe.