Dinosaur Extinction Habitable planets are in the minority, as your scientists well know. Habitable planets that can sustain life long enough for intelligent species to evolve are in an even slimmer minority. The reasons for this reduction is the number of catastrophes that can befall evolving life, setting the clock all the way back.
Catastrophes are many, and involve more than cataclysms.
In the main, evolving life falls because of the tiniest enemy, microorganisms which are likewise constantly evolving. Microorganisms have more flexibility than larger animals and greater mobility than plants. They can change hosts if their original host is not available. They have fewer factors to deal with. They can survive in water or air, most often, eat most anything at hand, divide to produce young and all in a day.