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.

Note that converting sunlight to electricity (what the 30% figure is about) is also quite different from using sunlight to convert CO2 to glucose (what the 6% figure is about).

Apparently the first stage of photosynthesis effectively does convert sunlight to electricity and does that with 90% efficiency, which is far beyond what our technology can achieve.

Source: https://www.economist.com/the-economist-explains/2014/12/03/...

It's also important to consider that photosynthesis only captures photons in a very narrow spectrum relative to the total spectral output of the sun.

But not all of the plant's absorbed energy goes into releasing oxygen.