Why didn't Imam Malik verify the authenticity of the hadith he cited as evidence?

Imam Malik is one of the Imams distinguished by piety, vast knowledge, sound intellect, precise reasoning, excellent inference, and authentic chains of narration. He built his madhhab (school of thought) upon principles derived from the objectives of Sharia, such as sadd al-dharā’i‘ (blocking the means), al-masālih al-mursalah (unrestricted public interests), istihsān (juristic preference), reliance on custom and tradition, the practice of the Companions, the law of those before us, istishāb (presumption of continuity), and the practice of the people of Madinah, in addition to the agreed-upon principles like the Qur’an, Sunnah, ijmā‘ (consensus), and qiyās (analogy). Infallibility is not attributed to him, and his acting upon a da‘īf (weak) hadith may have been an error, or based on other principles, with the permissibility of acting upon a weak hadith under certain conditions: that it pertains to meritorious deeds (fadā’il al-a‘māl), that its weakness is not severe, that it falls under an established principle, and that its authenticity is not believed when acting upon it, but rather it is done out of caution.