Because Allah the Exalted has said: "When any of you comes from relieving himself and you do not find water, then perform tayammum." ‘Ata says that if something like a worm or a louse comes out from a person’s back passage (i.e., anus) or front passage (i.e., penis or vagina), he should repeat his ablution. And Jabir bin ‘Abdullah says that if a person laughs during prayer, he should repeat the prayer but not the ablution. And Hasan (al-Basri) said that whoever shaves his hair, cuts his nails, or removes his socks (after ablution), there is no need for him to repeat ablution. Abu Huraira (may Allah be pleased with him) says that ablution is not obligatory except due to hadath (i.e., a state of ritual impurity). And it is narrated from Jabir (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) was present during the battle of Dhat ar-Riqa’. A man was struck by an arrow and much blood flowed from his body, yet he still performed bowing and prostration and completed his prayer. And Hasan al-Basri said that Muslims used to always pray despite their wounds. And according to Tawus, Muhammad bin ‘Ali, and the people of Hijaz, ablution is not obligatory due to (the flow of) blood. ‘Abdullah bin ‘Umar (may Allah be pleased with them both) pressed a boil of his and blood came out, but he did not repeat his ablution. And Ibn Abi Awfa spat blood, yet he continued his prayer. And Ibn ‘Umar and Hasan (may Allah be pleased with them) said regarding the one who is cupped, that he should wash the place where he was cupped, and there is no need to repeat ablution.

Narrated Abu Huraira: Allah's Apostle said, "A person is considered in prayer as long as he is waiting for the prayer in the mosque as long as he does not do Hadath." A non-Arab man asked, "O Abu Huraira! What is Hadath?" I replied, "It is the passing of wind (from the anus) (that is one of the types of Hadath).