Because
1. My memory from that time is crystal clear. I recall seeing words on a billboard and reading them.
2. After that, my grandmother, who lived with us then, taught me from old books, starting with prayers.
I too have crystal clear memories of reading whole words as a young child of four.
I don’t remember my mother teaching me the letters and sounds when I was an even younger child, but I do remember practicing reading with her and a McGuffy’s Reader.
Point is, I remember reading, I don’t remember learning to read. I only know because my mother told me.
If they are still alive, ask your mother and grandmother how you learned to read or how they taught you. You might be surprised at the answer.
I once had a very brilliant four year old in my pre-school that had learned to read watching Wheel of Fortune. His reading level was adult and he was doing 3rd grade math when he was brought to me.