Why do Muslims not believe in the Trinity?

يَٰٓأَهْلَ ٱلْكِتَٰبِ لَا تَغْلُوا۟ فِى دِينِكُمْ وَلَا تَقُولُوا۟ عَلَى ٱللَّهِ إِلَّا ٱلْحَقَّ ۚ إِنَّمَا ٱلْمَسِيحُ عِيسَى ٱبْنُ مَرْيَمَ رَسُولُ ٱللَّهِ وَكَلِمَتُهُۥٓ أَلْقَىٰهَآ إِلَىٰ مَرْيَمَ وَرُوحٌۭ مِّنْهُ ۖ فَـَٔامِنُوا۟ بِٱللَّهِ وَرُسُلِهِۦ ۖ وَلَا تَقُولُوا۟ ثَلَٰثَةٌ ۚ ٱنتَهُوا۟ خَيْرًۭا لَّكُمْ ۚ إِنَّمَا ٱللَّهُ إِلَٰهٌۭ وَٰحِدٌۭ ۖ سُبْحَٰنَهُۥٓ أَن يَكُونَ لَهُۥ وَلَدٌۭ ۘ لَّهُۥ مَا فِى ٱلسَّمَٰوَٰتِ وَمَا فِى ٱلْأَرْضِ ۗ وَكَفَىٰ بِٱللَّهِ وَكِيلًۭا ﴿١٧١﴾

O People of the Scripture! Do not exaggerate in your religion nor utter aught concerning Allah save the truth. The Messiah, Jesus son of Mary, was only a messenger of Allah, and His word which He conveyed unto Mary, and a spirit from Him. So believe in Allah and His messengers, and say not "Three" - Cease! (it is) better for you! - Allah is only One Allah. Far is it removed from His Transcendent Majesty that He should have a son. His is all that is in the heavens and all that is in the earth. And Allah is sufficient as Defender.

لَّقَدْ كَفَرَ ٱلَّذِينَ قَالُوٓا۟ إِنَّ ٱللَّهَ ثَالِثُ ثَلَٰثَةٍۢ ۘ وَمَا مِنْ إِلَٰهٍ إِلَّآ إِلَٰهٌۭ وَٰحِدٌۭ ۚ وَإِن لَّمْ يَنتَهُوا۟ عَمَّا يَقُولُونَ لَيَمَسَّنَّ ٱلَّذِينَ كَفَرُوا۟ مِنْهُمْ عَذَابٌ أَلِيمٌ ﴿٧٣﴾

They surely disbelieve who say: Lo! Allah is the third of three; when there is no Allah save the One Allah. If they desist not from so saying a painful doom will fall on those of them who disbelieve.

قُلْ هُوَ ٱللَّهُ أَحَدٌ ﴿١﴾

Say: He is Allah, the One!

قُلْ يَٰٓأَهْلَ ٱلْكِتَٰبِ تَعَالَوْا۟ إِلَىٰ كَلِمَةٍۢ سَوَآءٍۭ بَيْنَنَا وَبَيْنَكُمْ أَلَّا نَعْبُدَ إِلَّا ٱللَّهَ وَلَا نُشْرِكَ بِهِۦ شَيْـًۭٔا وَلَا يَتَّخِذَ بَعْضُنَا بَعْضًا أَرْبَابًۭا مِّن دُونِ ٱللَّهِ ۚ فَإِن تَوَلَّوْا۟ فَقُولُوا۟ ٱشْهَدُوا۟ بِأَنَّا مُسْلِمُونَ ﴿٦٤﴾

Say: O People of the Scripture! Come to an agreement between us and you: that we shall worship none but Allah, and that we shall ascribe no partner unto Him, and that none of us shall take others for lords beside Allah. And if they turn away, then say: Bear witness that we are they who have surrendered (unto Him).