What's "too subjective?" Well, take fashion, for example. Can you teach an AI to design clothes? Of course you can. Can you teach an AI to anticipate what will become trendy for the next year among teenagers? Probably not, just because it's hard to train a model to anticipate fleeting whims. You'd need someone who's really plugged into "the culture" to anticipate future trends, and that's more art than data science.
There's probably other examples I could give, but that's the one that comes to mind most.