The Overton Window works as a concept when we all talk to each other. But we now have three separate echo chambers - "Left", "Right", "Center" - and they each have their own Overton Window.
Its because the center failed to provide a path that people were satisfied with, hence there was movement away from the center into the fringes. It is entirely the center's fault that they are small. Maybe its just the end state of any sizable society.
The parties reshuffle themselves every few decades. Yes in the American system, it is inflexible to have more than two parties but the parties themselves are quite malleable and are following changing trends even though it may not appear to be so day to day. See the movement of Republicans towards far right despite the party leadership trying to stop it (they failed). The same seems to be happening on the Democratic side with them trending towards the far left although that might take a few more election cycles to bear out.
That's because the center doesn't work for many things.
If you support individual patients making their own healthcare decisions with their doctors, how do you find the center with someone who wants to take that right away for reasons that have zero effect on them?
If you support a consenting adults right to love and marry whichever consenting adult they want, how do you find the center with someone who wants to tell people which consenting adults they can and can't marry?
If you think people should not be denied jobs or housing based solely on what's in the their pants, how do you find the center with people who think it's okay to deny people jobs and housing based on what's in their pants?
How do you find the center with the Nazi party?
The center is meaningless for most issues that actually matter.
The actual definition of the Overton window exactly predicts what you both describe. See wikipedia for a better description.