Because it should.. because it's wrong!
Mr Tree offers the extremely common misconception:
There's a reason that this is on our list of Atheist Clichés That Need To Stop Being Said. It's so plainly dumb it shouldn't need explaining.
Like I said to the Aussie Atheist, what is being described when you say 'lack of belief' is agnostic non-theism. Atheism is specifically a person who believes or claims to know that God is not real.
If the word 'atheism' also becomes used to cover 'lack of belief' it becomes a useless word.
Dad: What flavour ice cream do you want?It's like that. It's unhelpful and gives no information. I really don't see what the problem is with using the correct phrase to describe yourself is. As far as I can tell, these atheists think that atheism is fashionable and want to be with the 'in' crowd. They aren't really fans of Will Ferrell's movies but they pretend to like them so people think they're cool.
Son: Ice cream.
The other thing that's handy for atheists who use the 'lack of belief' definition is that they don't have to defend it. It's not a claim, it's not an actual belief, it doesn't need evidence because it's the default.
I'd agree if they meant agnostic non-theism.
Lack of belief is a silly phrase anyway. I can turn it on its head to show how useless it is.
I lack belief in a godless universe. I have no evidence that God does not exist.Oh no! A double negative! Let's fix that!
I lack belief in a naturalistic universe. I have no evidence that naturalism is true.See? According to the belief lacking atheists, I'm now free of the burden of proof.
So if you are someone who uses this phrase, stop it. Call yourself an agnostic non-theist. It's more honest and it'll make your life so much easier.