Doctor Who Tie-Ins: 10 & Rose

I just finished all the Doctor Who books featuring 10 and Rose, books #7-12 in the numbered series. And I have to say I'm rather disappointed by their quality - or lack thereof. Mostly, I gave them 2 stars, one I haven't even finished because it bored me to tears. I wonder, with so many devoted fanfic authors out there, why are tie-ins given to write to professional authors who don't give a flying banana about the show/characters in the first place?