I don't know why they do that, but it's really the main (only?) drawback to KDP.
My guess is that they do expand, but only when genuinely needed for long-term. They probably keep a certain amount of blank books in stock, and ration them out to PoD books somewhat evenly, with established books getting higher priority.
This was the video that singlehandedly got me all those sales I mentioned earlier: https://www.tiktok.com/@alwayscandid/video/73180668430448755... ... honestly it's quite poetic how it all turned out. I did almost no work making a book, I made way more money than I ever deserved, I wasted all of it, and now I'm sleeping in my car. It's very fair, it's more than fair. It's generous.
In any case congrats that’s awesome. What was the book, if you don’t mind sharing?