Why the focus on Word Count for new works of fiction?

Just ticked over 47 words (47,111 to be exact) on my new book "Levels" (or "Lift" or "Climbers" ... still not sure what the heck to call it).
Why the focus on the number of words? Well, it seems that the industry uses word count as a sort of gatekeeper. This strikes me as both useful (until I get to a certain number certain publishers will not even look at the book) and, at the same time, terribly not useful.
Quantity is, of course, no indicator of quality. My first book, The Hare, The Bow and the Girl, was 33k words. Whether is was good or bad certainly was not reflected in that count. When I approached Amazon about it they flat out told me, within minutes of submission, that they did not look at anything under 45k words. Fair enough.
So I sat down and added about 17k words, or so, to get over the 45k hump. Doing that did not make the book better, in and of itself. But, I did feel that going back and revisiting the book after letting it sit for almost a year, allowed me to add to the book, clarifying some scenes, inserting other scenes that fleshed out characters, etc. The act of revision what was mattered here, not the act of added words.
I am still not entirely sure why 45k or 50k is some sort of magic number. That's under 150 pages (on a kindle). Most "serious" books I read are usually 200-300 pages, or more even. All The Turns of Light (Paths of Shadow Book 2) by Frank Tuttle is 250 pages, that's plenty more. The Fault in our Stars is 337 pages. Does that make TFIOS 1/3 better than Tuttle's book? Both are good books (although, to be fair, TFIOS is an instant classic, but if you like great steampunk fiction Tuttle's is cool as heck!)
Could it be that some stories really don't lend themselves to 150 plus pages? Are they less serious literature because of their length?
I don't have an answer really. As metrics go, word count is more useful than say number of chapters or page numbers (both of which are easily manipulated). Still, it feel like a strange way to approach fiction.
D H Richards