Why were the four madhahib established, even though the Ahl al-Sunnah are a single group?

There is no contradiction between the unity of the Ummah and the existence of jurisprudential schools of thought. The differences among scholars stem from various reasons in their evaluation of evidence, and these differences are not in the fundamentals of religion but in subsidiary matters, just as occurred among the Companions concerning the 'Asr prayer on the day of Banu Qurayzah, and the Prophet (peace and blessings be upon him) did not rebuke anyone. Therefore, depicting jurisprudential disagreement as a cause for disunity is an error and ignorance.