This practice is throwned upon by moderators of the Physics SE, but it turns out to be quite common elsewhere, e.g., in the biology community. The reason is that they adhere to much more rigorous standards for the answers - requiring them to be nearly at the level of scientific publications and including citations (if not, they are mark by a moderator). Thus,...
Let us not exclude this explanation (quote from one of the links):
Comments cannot be downvoted. For unclear questions, if the question
is clarified or other people interpret the question differently then
people might downvote an answer that does not answer their
interpretation of the question. Also someone may be afraid of losing
rep from an answer that ...