You must be unlucky I do not see them often but I have encountered carriers a few times my best kill was a Fuso though then after I got him I have not seen an IJN BB in two years of playing since.Maybe all these others are sinking the good stuff before we find them.
It is interesting that many new players seem to run into the Yamoto or a CV and get a kill maybe that is why some say they would rather be lucky than skilled any day.