Because they're super packed with students that do whatever. In this particular case, WSU Sigma Nu did not "burn down" as stated in the video. There was a fire, caused by a candle, in the third floor of the left add-on section of the house. Fire damage was relatively minimal. However, as is often the case with fires, water damage was extensive. Building codes required that rebuilding had to be done to modern code. The add-on section was built in the '60s, and the main house in the '20s, so extensive work was needed to rebuild what was damaged. Since so much had to be redone, and the house was rendered uninhabitable, the decision was made to rebuild the whole house. Shout out to the group of alums that organized the effort, including the fundraising to the tune of $2MM+. Some work has been done since of course, but what was done during the rebuild served as groundwork for what you see here - one of the tops in the nation.
