Because generally, boundaries are generous for people to get an A, so many people get an A. It's just not considered 'WOW' anymore!
It is generally the case on TSR, as so many people here aiming to study medicine at oxbridge, they need a solid set of A*s at GCSE, so they just see anything below a fail.
I will be lucky to get 3-4 A*s for GCSE, so imagine how I feel when I see everyone here predicted nearly all A*s. It makes me feel like a mediocre student, which I know really I am much higher than mediocre, but still!
A mediocre student is someone with a few As, big chunk of Bs, and one or two C's.