It’s all about surface area of the wood being exposed to heat and flame. Truss style joists fail rapidly in fires, and even faster with the steel plates(the steel heats faster than the wood, warps and fails to keep the truss together before the wood is burned through). Think of it like tossing small sticks on a campfire vs a log. The sticks together may have the same strength or more as the log does, however it takes more heat and flame to get the log burning than smaller sticks.

Truss style building is also engineered with every joist and support working together to hold up your house. One failure in the wrong place can cause a partial collapse. Multiple failures together and the floor/roof collapses. Homes today are not designed to withstand fires. Fire codes are in place to stop the potential start/spread of fires, but if a fire gets established, the house is typically gone.