Why does Sharia stipulate the presence of a mahram for a woman during Hajj and Umrah, even though necessity may not always require it?

All that Allah has commanded encompasses benefits and wisdoms that may be hidden from some and apparent to others. A Muslim must submit to Allah's command even if he does not know its wisdom, because the Shari'ah only commands what contains benefits for the servants and only prohibits what entails their corruption.

A woman should not travel except with a mahram. This is for her protection and preservation from evil and corruption. The mahram surrounds her, protects her, and takes care of her affairs during travel, which is a place of fatigue and hardship. Due to her weakness, a woman needs someone to support her. The shortcomings of some men or the existence of rare cases do not change the ruling, because the consideration is given to the general prevalence.