Because I learnt to crochet at an early age, and did my very first granny square by deconstructing one of my aunts from the blanket she made me. I had no idea what the stitches were called. Then I had some help from 2 very elderly sisters who'd been missionaries in China. They showed me new stitches but didn't tell me their names or how to read a crochet pattern.
So they only way I could crochet anything was to make it up as I went along and use a measuring tape. I only very recently learnt the name of the stitches and in June I taught myself to read a pattern in order to help a young friend who was trying to use a pattern to learn to crochet.