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."