Apart from those who generated css dynamically. And those who hated the situation enough to create preprocessors to improve their lives.
"Before X people were ok without X" is a tautology.
No, it's not.
Sometimes new techniques appear without anybody feeling there was a problem in the first place.
Other times they appear after people struggle and curse their way without them.
"Before X people were ok without X" only refers to the first case, so it's not a tautology applicable to all cases before X.
E.g. people always struggled with webpage layout, and knew they needed a solution X to appear, even before an X emerged.
Using tables for layouts was once state of the art as well with no complaints. Things change, as do best practices. You can use inheritance to reuse values without preprocessors, or you can switch to preprocessors and use variables and mixins for composition. The question of which is better (inheritance vs. composition) is a personal one, I prefer composition.
If life was fine without preprocessors, then why did they see such rapid adoption?
I'm not arguing for or against CSS preprocessors here. I'm trying to point out that one shouldn't have difficulty imagining coding a site with regular css. It's every bit doable, it has been done widely in the past and it is in fact still done today.
In some cases - perhaps many - it's not ideal, sure. But when OP says that not using these tools is unimaginable, I see that as a lack of effort on his/her part into trying to understand the point of view of the people who don't use them.
If I was a LESS user and somebody said, "I can't imagine a case where you'd want that tool", I would scoff at them because there are plenty of examples out there (and in this thread), and their statement shows that they're really not even trying to understand my point of view.
If they instead said, "I can understand why you think you need that tool, but ..." then I wouldn't have a problem because they have made an effort into understanding my point of view (and they can at least imagine using my workflow in their head, even if they don't like the idea).
This may be pedantic - after reading OP's post again, it seems possible that OP wasn't being literal about the imagination thing. But that didn't occur to me at the time of writing.
It wasn't that long ago that nobody used [the current big-name] CSS preprocessors. But site admins got along fine, and I certainly didn't hear many complaints.
To each their own, but I don't find maintaining a big site with 'regular' css unimaginable.