Building would have had to have been rigged on the exact same floors the planes crashed into. The explosives would have had to been undamaged and intact after the impact. That removes the use of most conventional explosives, which would have been melted and neutralized by the fires. Also would have required piloting skills not held by any of the hijackers. So fantastic (see non existent) explosives were used and the planes weren't flown by inexperienced terrorists.
Simple answer is better. Angry Muslims attacked some buildings.