The majority of failures with rolling bearings can be detected in an early stage with careful vibration analysis. That is a common basis for not monitoring temperature.
However lubrication conditions for greased bearings are better judged by temperature. Abnormal loading conditions which will eventually degrade the bearing may show in temperature earlier than the bearing degradation does. And often temperature is continuously recorded/alarmed while vibration is done by walk-around routes which occur only once a month or so.
Choose a strategy based on your reliability / cost tradeoff. (there are of course many more factors - criticality of the machine etc).
