Because the characters are different enough that it causes actual day-to-day problems (I see text all the time on printouts/signs where some incorrect font substitution has taken place and you get Chinese instead of Japanese characters). You can (probably correctly) argue that these are really just deficiencies in text editing/markup tools where you can't mark the language of text but the fact that that's required for correct presentation only in CJK languages seems to indicate that it's a problem with this unification in particular.

How would you like if there was Latin/Greek/Cyrillic unification? I mean they're all just alphabets that make the same sounds they're all basically the same right?