There are two reasons, IMO: tradition and ease of assignment of material. Tradition needs little explanation as the history of the connection between learning psychology and language teaching reveals various theories of how the forms of a language can be uploaded into the learner’s brain for later use in communication. Many of those theories have not worked out well and the result has been a paucity of communicative ability on the part of learners. In a word, the field is littered with bodies.
But the second reason goes back to what Diane Musumeci pointed to in her Breaking Tradition where she found letters to curriculum designers in the late Middle Ages citing the need for structured material to keep the students under control. The open-ended dialoguing needed for acquisition allowed for too much slippage and the teachers demanded tight lesson plans, thus destroying the communicative atmosphere the brilliant educators like Comenius, Loyola, and Guarini (??) urged. The latter were forerunners of the acquisition via comprehensible input school of language teaching. Anyone who has give tprs a good try (see my blog category tprs for my own recent experience) knows it is hard and requires firm control of the class. Given the current trends in our schools, no one is about to give a creative and innovative teacher the permission, the freedom, the time, or the professional status to allow there to do anything other than test prep.