[Professor Willingham cited the research of Mark Changizi for this explanation.]
“A reasonable hypothesis is that the visual system has been tuned over time (either evolutionary time, or the lifetime of an individual, or both) to best perceive shapes that appear most frequently in the environment. People who invented alphabets unconsciously capitalized on that property of the visual system. The shapes that people see most easily were judged to make nice letters. It’s a good example of what we mean when we say that the brain is not designed for reading and writing—rather, we co-opt existing mental mechanisms to make literacy work.”
Excerpted from: Willingham, Daniel T. The Reading Mind: A Cognitive Approach to Understanding How the Mind Reads. San Francisco: Jossey-Bass, 2017.