One reason is that the max_size function isn’t very useful at all, and the committee doesn’t think it is worth the trouble to try to fix it. So it is just left the way it is, because it is part of the documented interface.
See library defect report #197:
http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2012/n3440.html#197
max_size() isn’t useful for very many things, and the existing wording is sufficiently clear for the few cases that max_size() can be used for. None of the attempts to change the existing wording were an improvement.