That's a question for a web developer, which I am not. I would expect it to be the max common resolution width. A quick Google shows that modern ultrawides are 5120x1440. So that's probably why.
Lemmy does not use the entire screen width. The way it has been embedded in the page means that image takes up only 850 pixels of horizontal space so it could be 5x smaller and no one would be able to see the difference.
Lemmy really should be automatically resizing the images (on the server) when they are uploaded, not every single time the community is viewed (in the browser).
it's not. the lemmy-ui max width for the poster element is far smaller than that (1104x960). in fact, the poster element is set to be a near-square (displays as rectangular in web and mobile web on the page header), as it also displays in the sidebar and in mobile apps as a square if the image is. most mods simply assume it's a rectangle and upload a rectangular image.
this image is made to be the largest usable resolution lemmy can display as a community poster and optimized to be very small in file size. see on https://lemm.ee/c/plex