Because Allah, the Mighty and Majestic, is towards the face of the one who is praying as long as the worshipper remains attentive towards Him in his prayer.

It is narrated from Sayyiduna Ibn Umar (may Allah be pleased with them both) that the Noble Prophet (peace and blessings of Allah be upon him) saw phlegm in the direction of the qiblah in the mosque, so he (peace be upon him) rubbed it off with his hand or said that he scraped it off with his hand. Then he turned towards the people and expressed severe displeasure at them. And he said: "Indeed, Allah Almighty is before the face of any one of you in his prayer. Therefore, let no person ever, ever spit phlegm in the direction of his face during his prayer."

Sayyiduna Abu Wa'il رضي الله عنه narrates that when Shaith bin Rabi'i prayed standing next to Sayyiduna Hudhayfah رضي الله عنه, he spat in front of himself. Sayyiduna Hudhayfah رضي الله عنه said: Indeed, the Messenger of Allah صلى الله عليه وسلم forbade us from this and said, "When a person begins prayer, Allah Almighty turns towards him with His noble face and does not turn away from the worshipper until he himself becomes inattentive to Allah Almighty or his ablution breaks."