It's more like the conditions required to support an overclocked CPU can't be taken for granted. For 4.2GHz on an i7, on the typical chips (there are golden ones made of good silicon and those get sold as the $1000 chips), you'll need good cooling (either a $90 D14 or $100+ custom water cooling) and a fairly decent motherboard as well to supply clean, stable power to the CPU for it to be able to run at 4.2GHz. Not to mention airflow across the motherboard to cool the power delivery components, a good power supply, etc.
When Intel makes these CPUs, they have to account for the people that are going to put them in $80 motherboards, or for OEMs who are going to put them in $20 motherboards (99.9% of their market in fact), and also for their own included stock coolers.
Still, it's not impossible if they use the very best chips to get a lot more out of it than a normal CPU. For example, the AMD Phenom II X4 965 runs at 3.4GHz and it meets the factory specifications for heat and voltage. That's pretty much as high as a lot of people overclock their CPUs. So you can also overclock a Phenom II X4 945 to 3.4GHz but you'll need the good cooling and motherboard to support it, whereas a 965 can do it wherever.