Very simply, a power conditioner fliters a small amount of high frequency noise on the AC line while a Power Plant generates new power free of any noise or problems.
The easiest way to think about the difference between a power conditioner and a Power Plant is using a water filter as an example.
A simple water filter is capable of removing some of the dirt found in water. Simple water filters are not capable of turning dirty contaminated water into pure clean water.
To convert dirty, contaminated water into pure and clean drinking water, it is necessary to first boil the water and convert it to steam; then reversing the process, convert the steam back to water leaving all the contaminants behind.
In the same way, a Power Plant converts the AC to pure DC, then back again to pure AC leaving any problems behind.
A power conditioner cannot perform these functions and can only have a minor improvement on the AC power. The basic problems in all power lines remain with a power conditioner: unstable voltage, dynamic loss, distortion, noise etc.