It's kind of an unsatisfying argument, but what I've heard (back in high school biology) is that the reason you see the relatively low efficiency is because that's all the plants need. In the course of evolution there was either not a great enough reward for more efficient photosynthesis, or no mechanism developed that was able to utilize something like 30% of incoming sunlight.
It's also important to remember that plants evolved to reproduce and spread as effectively as possible, not to be as photosynthetically efficient as possible. Engineered machines, however, can be designed with energy efficiency as the main goal.
It's also important to remember that plants evolved to reproduce and spread as effectively as possible, not to be as photosynthetically efficient as possible. Engineered machines, however, can be designed with energy efficiency as the main goal.