A thousand good deeds cannot amount to even a single bad one.
That's true. But let me give you a more serious answer:
The fact that improvements are less obvious to readers is the timeframe. Bad chapters that fall off the cliff are a big step down. (They "fall" down after all.) Positive improvements, especially over the course of a long-running series, are more incremental and become "the new standard" before they become big enough to be seen as a clear step up.
So the majority of readers won't realize on a chapter by chapter basis how you improve. They'll just remember you as a writer with a better skill level from then on.
The clear and open comparison are either re-reads or new readers who binge-read through your stuff. They shorten the timespan between chapters so much, that the upward trends becomes a new cliff big enough to be seen. Obvious signs of that are the usual "it gets better after chapter 50" you see in many webnovel reviews or comments toward negative reviews. It's not that chapter 50 or 100 had that one big event that jumpstarted everything, but the slow increase of quality over all those chapters resulted in a better product at that time. That's especially true for stuff like flow in chapters and the overall story.
Most writers don't show "obvious improvements" in a short time. Especially, if you don't start at zero. That's just not a realistic trajectory if you gave it your all from the start. So most readers also won't see those improvements as obvious. Simply because they aren't there. It's the reason you won't see readers go "Oh, your dialogue has improved so much" every few chapters. Because it doesn't. That said, those incremental improvements are still noted subconsciously and that also counts in your favor.
It's just that the idea of "improving a lot in a short time" isn't really that realistic...