Because where is the return of PCV ?
Into the intake manifold with air and oil fumes.
That is why a PCV has a spring loaded pintle valve to control when and how open that flow path is.
The more blowby being sent into intake that air/oil will effect in the end what the front O2s report and of course the more oil fume volume the worse the charge in cylinder.
Also the smaller the hole is as in orifice type the more unburned fuel and oil fumes end up in cylinders to reburn, cause backfire and changing AFR

Here is a small filter I just stuck in to do some testing of a filtered small can in series with PCV and intake.