You need to understand only three things to understand why.
1. Steel is made by heating it up and shaping it. The buildings were on fire and the steel structures weakened by head and the loads on them.
2 Gravity wins. It is ever present and is always in the direction downwards.
3. The buildings were designed well and aircraft commission was one of the things considered.
No matter how well built or designed a structure that is hot with a fuel bomb is likely to collapse. The attack was planned with build collapse in mind.
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