Why is no operating temperature range specified?

Discrete semiconductor devices do not impose any restrictions for operating temperature.
This is because, unlike in the case of an IC, the heat generated in the element varies depending on the usage condition of the customer. The channel temperature of the device is calculated from voltage, current, etc. applied to the device. For discrete semiconductor devices such as MOSFETs, the maximum value of this channel temperature is specified.
Although use at the maximum channel temperature T_ch (max) or less is possible, reliability such as deterioration, life span, etc. must be considered. Degradation, such as performance degradation, accelerates as channel temperature rises.
In order to maintain stable performance for a long period of time on your equipment, please design with derating to the maximum channel temperature T_ch (max).

For information on derating, please refer to the following document.
Document title “Concepts of Derating”

• Reliability Information “Handling Precautions and Requests”