There are a number of reasons why the Arab armies failed in the Six-Day War in 1967.
Disunity: The Arab states were not united in their goals or strategy. The Egyptian president, Gamal Abdel Nasser, wanted to retake the Sinai Peninsula, which Israel had captured in the 1956 Suez Crisis. The Jordanian king, Hussein, wanted to protect the West Bank, which Israel had occupied in 1948. And the Syrian president, Hafez al-Assad, wanted to expand Syria's territory to the Golan Heights. These different goals and objectives made it difficult for the Arab states to coordinate their military efforts.
Underestimation of Israel: The Arab states underestimated the strength of the Israeli military. They believed that Israel was a small, weak country that could be easily defeated. This underestimation led to a number of strategic mistakes, such as the Egyptian decision to concentrate its forces on the Sinai Peninsula, which left its other borders vulnerable.
Technical inferiority: The Arab armies were technically inferior to the Israeli military. Israel had a more advanced air force, navy, and armored corps. The Arab states also lacked the training and experience of the Israeli military.
Lack of strategic depth: The Arab states had little strategic depth, which made them vulnerable to Israeli air attacks. Israel was able to bomb Arab airfields and communication centers, which crippled the Arab armies' ability to coordinate their operations.
Israeli surprise: The Israeli military launched a surprise attack on the morning of June 5, 1967. The Arab states were caught off guard and were unable to react quickly. This gave Israel the upper hand in the war.
As a result of all of these factors, the Arab armies were soundly defeated in the Six-Day War. Israel seized the West Bank, the Gaza Strip, the Sinai Peninsula, and the Golan Heights. These territories remain under Israeli control to this day.