Why are so many 9/11 conspiracies adamant on “jet fuel cannot melt steel beams?”

giant planes moving at massive speeds rammed right into skyscrapers (and the pentagon too i guess). it's pretty obvious theyd collapse regardless of whether bush or bin ladin was behind it. if the American government was behind 9/11, why would they go through the effort to hide thermite in the towers and fake planes crashing instead of just hijacking some 747 and pretending be part of al-qaeda? it just seems like an unnecessary complication in a (comparatively) straightforward conspiracy.