And why is that? Oh, it must be because a $2600 L-series superzoom lens isn't an 'actual zoom lens'.
Well, it's a zoom, but not quite lens. Although a decent performer in "ultrazoom" class, it is by far not impressive in "lens" class.
A combination of 24-85mm and 70-300 4.5-5.6 performs very similarly for a fraction of price. 24-70 + 70-300 L kicks it's ass big time for +500$. Better still, a set of primes would cost less, offer better control over DoF, and even better quality.
As a matter of fact, I'm quite sure there hardly is a real-life discernable difference between pictures taken with 28-300 and top of the line compact.