Why is a man's awrah (private parts that must be covered) from the navel to the knee and does not include the chest? And why did the Prophet prevent Aisha from sitting when Ibn Umm Maktum visited him, even though a woman is affected by touch and a man by sight? And why was the verse enjoining lowering the gaze revealed for women?

First: It must be noted that the hadith mentioned in the question is not authentic, as it is weak due to an unknown narrator in its chain of transmission. As for the ayah (verse) mentioned in the question, it is a combination of two ayat: Al-Ahzab/59 and An-Nur/30-31.

Second: It is permissible for a woman to look at a man who is a non-mahram to her, provided that it is not with lust and that there is no fear of fitnah (temptation) arising from it.

Third: The fact that a man's awrah (intimate parts) extends from the navel to the knee does not mean it is permissible for him to expose his chest in front of men or women. Exposing the chest is a breach of muru'ah (chivalry/propriety). If a permissible act leads to harm, then it becomes prohibited. Thus, if a man exposing his chest causes fitnah, then it is prohibited.

Clothing has been legislated for numerous reasons, including: conforming to fitra (innate disposition), adornment, protection from heat and cold, and covering the awrah. Allah has bestowed a favor upon His servants by creating clothing that covers their private parts and has drawn attention to another, greater garment, which is the garment of taqwa (piety) - (O Children of Adam! We have bestowed upon you garments to cover your private parts, and as adornment; but the garment of taqwa - that is best).

The garment of taqwa is better than physical clothing; it is the beauty of the heart and soul, and it remains with the servant. As for outward clothing, its purpose is to cover the visible awrah or for beauty. The absence of the garment of taqwa exposes inner awrahs and brings about disgrace and scandal.