It was everywhere. The smell of stale cigarette smoke was in nearly every public space. This was in the 80s in the US, so smoking was already in decline, but the smell was still this constant background presence.
If dad didn't smoke, surely he had guests who did.
And in the late 90s, being on a plane and the chairs had a metal folding door on the armrest that exposed an ash tray. Smoking on planes was already gone or going away, but the hardware lingered for quite some time.
If nearly everyone smoked, then even nonsmokers were constantly getting a fair amount of secondhand smoke.
This would raise the background rate of cancer, making it appear that smoking raises your risk by less than it actually does.
[0] https://en.wikipedia.org/wiki/Roy_Castle#Illness_and_death
If the "normal" rate of lung cancer is X, the observed rate in nonsmokers who get secondhand smoke is X+Y, and the observed rate in smokers is X+Y+Z, if you compare nonsmokers and smokers it looks like smoking increases your rate by Z when it's actually Y+Z.
No, that's where you're wrong.
You are only interested in that independent risk.
I, and many others, are interested in how much smoking changes that risk.
Picking random numbers, let's say smoking gives you a 10% chance of lung cancer. It's fine for you to only care about that 10% number, you get to care about what you want to.
But for the rest of us, when making informed decisions based on risk, it matters whether smoking changes it from 9.9% to 10%, or 0.1% to 10%.