First-Order Lowpass RC Filter
It's all about recognizing the reality of parasitic components. 

For instance, an 0603 style resistor might have an equivalent parasitic parallel capacitance of 0.2 pF (in situ on a PCB). At high frequencies, this will form an AC potential divider with C1 (10 pF) and, in terms of attenuation, is ~0.02 or, -34 dB and nowhere near -100 dB as would be implied if the parasitic capacitance of the resistor were ignored.

You can also argue that the parasitic series inductance of the 10 pF capacitor makes it less than ideal and this also makes the attenuation of 100 dB very unreasonable. All in all when you get much above several hundreds of MHz you cannot expect the sort of performance you'd get at audio frequencies.