Because it’s a bad textbook!
Most textbooks are about 20-30 years behind the state of the methodological art. Prior to writing The Essential Guide to Effect Sizes I scanned more than 30 texts published between 2000 and 2009. I found that 9 out of 10 had nothing to say about effect sizes. If effect sizes were mentioned, it was only in passing.
A typical text book will show you how to assess the statistical significance of a test, but not how to establish substantive significance of their results. It will talk about p values but have little to say about effect sizes.
This will change. In the future textbooks will increasingly show students how to: estimate the magnitude of observed effects, gauge the power of the statistical tests used to detect effects, and interpret effect size estimates in meaningful ways.