We teach math, too.
Problem is, a lot of countries have caught up and in some cases, surpassed us. Many countries outspend us relative to their cost of living, their educators are paid much better and held in much higher regard, and their schools (K-12 equivalents, universities, trade schools) are funded at a much, much higher level. Most of the best performing countries spend more than we do, and shockingly, this corresponds to better results.
Additionally, while the literacy rate in the US has more or less remained the same, the literacy rates in many developing countries has exploded. Japan and Korea, for example, have eliminated their gender parity in literacy, and gone from feudal states to highly advanced countries. We have lower literacy scores than Australia, Canada and Japan, are on roughly equal footing to the UK, and better than Italy, France, and Poland. Most of the countries with functioning governments realized a long time ago that a literate society means a prosperous society.