Technically, they are introduced, but typically under the auspices of other subjects, namely fractions.
In order to use ratios and proportion effectively, one has to know fractions and their algorithms. Fractions and fraction algorithms are covered in 3rd Grade in the Common Core. In order to perform Fraction algorithms, one has to know basic arithmetic, which is typically built in grades K-2. That is why ratios and proportions are not developed until the underlying concepts are developed.
By 4th grade and up, the arithmetic, and fraction skills needed to effectively operate ratios and proportions are established, and then students apply fractions and algorithms in Ratios and Proportions.