The towers calapsed because steel loses the majority of it's strength when heated to the temperature of burning kerosene so the structure where the fire was could no longer support the weight of the tower above it.
It's as simple as that.
Things like the fire protection on the beams is irrelevent as even if it was still intact, it wasn't designed to take that level of heat for that sort of duration and extent.
It wasn't the aircraft fuel that caused the collapses. This burned off in a few minutes, being vaporized when the planes hit. The intense, enduring fires that followed was caused by Class A combustibles: wood furnishings, drapes, carpets and other textiles, paper etc,
ignited by the fuel.
Although the impact did blow off the sprayed-on fire retardant on the vertical structural beams, it was not these that failed. It was the much smaller transverse joists holding up the floors that weakened from the heat, pulled away from the verticals and began the progressive collapse. With the benefit of hindsight, the design had a fatal flaw as the vertical and horizontal beams were interdependent, in a kind of box structure. Nobody, when the building was designed in 1970, could have been expected to provide for the impact of a fuel-loaded jumbo jet.
Interestingly, there had been a prelude to this in a high-rise building, Ronan Point, London, England in the 60s, in a kind of reverse way. A gas leak on an upper floor caused an explosion that blew out the "curtain wall" on one side and the floor beams, now unsupported at the ends, collapsed and pancaked down all the floors, just as the WTC buildings did.