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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.
I really do not think that there is a hard and fast reason why they are not. It is probably due to the fact that ratios and proportions involve fractionas. Fractions are touched upon in the lower grades but are not used for calculations for some time. As a high school teacher, I believe that you could introduce ratio and proportion in the younger grades--especially if you are using a scientific or graphing calculator that allows the user to enter fractions. It might even be a good way to introduce fractions.
Most high school students are petrified of fractions and I think that is because they never became comfortable with them in elementary and middle schools. Focusing more on fractions and decimals at a younger age might help to make the transition to more advanced math smoother.
I believe that not understanding the "concept" of a ratio, in particular, is why Algebra and more advanced math is not understood. If you ask people in general what Pi is, they will not know because they failed to understand the concept of a ratio. I feel understanding the concept of a ratio is the most important idea to grasp. Without it, you will just never get anywhere in math courses.
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