They recovered four, three of them AT the crash sites! How. The black boxes didn't even survive. 
Jet fuel doesn't melt steel beams. But metal loses it's strength while retaining its solid phase as it is heated. https://www.engineeringtoolbox.com/metal-temperature-strength-d_1353.html
This is the issue with laypeople speculating on complex engineering failures. The mechanisms of action that one might intuit from looking at a complex systems failure and the ones that actually cause it are oftentimes not related to or even opposite.