Because just like weaning weights and yearling weights, the actual number would shock you.
Honestly, although I think the above statement is true, the politically correct response is: by making any number, including BW, WW,YW, "adjusted" the numbers that show can be directly compared to each other on equal footing.
Where as if they are not "adjusted", the publisher assumes the reader (buyer) is too stupid to make their own adjustments due to DOB, Cow, Breed, etc....
Also while reading catalogs, make sure all the information is either "actual" or "adjusted". I have seen some that go back and forth, publishing whatever looks best. So....KEEP YOUR EYES OPEN.
Kinda like when semen catalogs "omit" frame scores, WW, etc, on some bulls, but have it listed on others!!
Note: Remember on BW's......Round hole, Square peg.....size isn't all that matters!