As far as I understand, a chip in general and a MOSFET in particular has a maximum operating temperature (let‘s say 125 °C or 150 °C for Si) because above that temperature the risk increases of building a thermal runaway that creates a positive feedback and destroy the device.
But why is there a maximum storage temperature? For instance on this datasheet of the 2N7002:
Let‘s say the maximum storage temperature is 150 °C. What happens if a put a bare die chip (not mounted or anything) in a temperature chamber and heat it up to 200 °C for one hour? If the chip doesn‘t conduct current then there is no thermal runaway and since it‘s only the chip then there is no thermal stress with solder and substrate. Furthermore, this devices are normally rated to endure some (around 3) reflow profiles, which have a peak temperature (normally above 220 °C) which is above the maximum storage temperature.