Why harvesting energy why not harvesting power?
To answer the question, it's called energy harvesting because in most systems the rate of collection (input power) is very different from the rate of use (output power).

Most such systems run intermittently, so a key parameter is how much energy the the application consumes each time it runs (output power times run time). This then determines how often the application can run, because it takes a certain amount of time to collect at least that much energy (input power times collection time) in the energy storage device (capacitor or battery).