because it is the characteristic of F1 that separates it from other major racing championships.
lol, not all forms of open wheel series are directly related to F1.
F1 is the epitome of open wheel racing, but that doesn't mean the word "open wheel" directly equates to F1.
Hinting that "open wheel" is strictly regulated for F1 (or identified as F1), well.... all I can say is such suggestion is completely and utterly ludicrous.
Your statement implicates that F1 basically represents open wheel racing, and that all forms of open wheel racing are either F1, or F1's direct feeder series.
Seriously, do you honestly think open wheel is strictly for F1? What about LMP? Super-Karts? Hill climb? There are various different types of motorsport that share the open wheel design and regulation, but have no connection with F1 whatsoever.