Why is the Holy Kaaba open to the sky and not covered?

The Holy Kaaba has been roofed and not open since ancient times, a fact that is well-known and observed. A number of scholars, such as Al-Azraqi and Ibn Hajar, have mentioned this. Moreover, it was on the roof of the Kaaba that the polytheists hung their unjust document to boycott and besiege the Prophet, peace and blessings be upon him.