Because they were built by infidels.

Via Heavy:

In a new video purportedly released by the Islamic State, ISIS allegedly blows up the Temple of Nabu in Nimrud, aka Kalhu, Iraq. It then states that it’s its religious duty to destroy the Egyptian pyramids and Sphinx. The video is titled “The Axe of Ibrahim, the Intimate Friend of Allah” and it was released on June 7 on ISIS terrorist channels. It comes from “Wilayat Ninawa,” or “Nineveh,” the ISIS-held territory that includes Mosul, Iraq.

The video begins with footage showing a militant promising to destroy the temple of Nabu in the ancient Assyrian city of Nimrud. Further footage also shows the destruction of the reconstructed Adad and Mashki Gates and a large portion of Nineveh’s fortification wall, which was released as a photo report on ISIS channels in mid-May.

The last part of the 10-minute video shows the famous Great Pyramid of Giza near the capital of Cairo, Egypt. According to The Daily Mail, an ISIS militant then promises to blow up “ancient sites built by the infidels.”

ISIS believes any religious structures built prior to the alleged revelations of Muhammad are blasphemous. Muhammad had his first revelation in 610 CE. Modern civilization is about 6,000-years old.