I always assumed it was because of dilution of fuel or shearing. But I don’t know.
All I know is that I had one particular car where it was very noticeable. A Lexus LS460. Up to 4,000 miles it would appear to use nothing, not a drop...then when it reached 5,000 miles, BOOM. It would use a quart every 1,000 miles on the way to 7,000 miles. And then I’d change it. I experienced something similar in my 2018 Silverado.
I’m not sure why it happens - but I can tell you why it happens with brake pads - with pads the first half of the pads life will last longer than the second half, because of the thickness. The thickness of the original pads dissipates heat better, whereas the worn pads can’t, and will wear much faster because of what it no longer has (material thickness). Next thing you know, you’re metal to metal on the rotor. You thought you had as much time as the first however miles you put on those pads, but you don’t.