Why are explanations so hard to come by?

The question is inspired by my coming across an explanation for why plants are green. This should be common knowledge, universally taught in school. It seems to me that there are a lot of things simply explained that just never get taught.

For anyone interested, green light is reflected and rejected by plants. Light at both higher and lower frequencies is absorbed and used for photosynthesis. The frequencies chosen are based on their availability as a source of energy, which in turn depends on the energy per photon and the proportion of light occupied by a particular frequency in the spectrum as filtered by the atmosphere.

Light at higher frequencies than green are a relatively small proportion of the total available light, but the light is of higher energy. Light at lower frequencies than green is lower energy, but is highly abundant. Green light is in the middle in terms of energy and abundance and it turns out that the particular combination does not provide as much energy as the lower and higher frequencies.

Now how difficult is that to understand? Is there any reason why this should not be common knowledge?

Here is another one from botany that I recently learned. Do you know why it is difficult to grow plants in soil that is salty? Roots use a salt concentration gradient to draw water. It is a neat setup. Ordinarily the salt would go from high concentration to lower. However, the roots trap the salt so water moves from low salt concentration to high salt concentration in the roots. If the soil is salty then the whole system is disrupted.

Here is another example. I wonder how many people can explain why (apart from leap years) their birthday next year will be celebrated on the following day of the week as it was this year. The answer requires only very basic arithmetic.