I just like having the entire collection of files in one place, some of them are textbooks or reference books that don't sit well in either app entirely, and while I also keep novels there in a separate subcollection, most of my ebooks are non-fiction, where topics are the same or related to the rest of material I already keep in Zotero.

Some are non-academic PDFs, blueprints, guides, whitepapers or manuals that I refer to for a given project, and while this is not the main purpose of Zotero, as it is a research-oriented tool, it still proves more useful than other tools, tidier than classical folders+files, and less of a hassle than coming up with my own system using something like Obsidian's dataview plugin.

Calibre I prefer it to fix the metadata of my ebooks, but the local reading story isn't ideal for me. I've tried serving the collection over self-hosted web apps, to keep the page progress like calibre-web, but it wasn't ideal for me or conducive to what I look for: https://github.com/janeczku/calibre-web