+1 I totally agree with you, it is a good practice to write a descriptive messages and it is something that we have to try to do.

But I think that's why we are using Hamcrest and writing assertion in a pseudo-natural language (using asserThat, is(), not(), equalTo(), etc) an not another common assertions, because this library generates for us a compressible message for the developers.

For instance, if you write this assert:

assertThat(6, is(not(greaterThan(5))));

Hamcrest generate for you this message:

Expected: is not a value greater than <5>
   but was <6>