For instance, quite a few times I'll open a page in Google's results, scan the content and go back to compare it to another result. That doesn't necessarily mean I hate the first page or think it's terrible, just that I wanted to see what it had to offer.
It can also mean I did find the answer there, and am returning to Google to search for something else instead.
It's tough to distinguish cases like those from 'the content was bad' and 'the content was not what I was looking for'.
There's also the worry that it might end up hurting user incentives too, or alter the way in which users browse the internet as a whole.
After all, since finding out that YouTube rewards/punishes creators based on watch time/how much of the video someone watches, I'm now hesitant to even watch many videos at all, since I know some guy's career might potentially be put on the line.
Perhaps Google worries other people will be the same, and stop clicking as many results knowing that Google might punish the affected websites for not staying long enough.
They could hook this into Google Analytics and subtract the time you spent with that tab/window not focussed from your session duration.
I do fundamentally agree that session duration may not be the worst proxy for worthwhile content or, at least, engaging content, snd I think Google may use it as such.
Maybe this does happen but it not weighted highly enough against SEO.