This is probably because 1) most of the tests are used to grade test takers, i.e., placement tests; thus, they are treated as private information that should not be leaked, and 2) the primary focus of most language-educators is how to measure their students' ESL vocabulary, rather than the test results of the other test takers.
we did not include English proficiency tests popular almost solely in Japan, such as the "Eiken" test, an English proficiency test popular with Japanese high-school students while we required our test takers to have a TOEIC test score, it did not matter when they took the test.