Outlook makes sense because they use React Native in their app. If you look at their License sections they are using a ton of libraries which is common.
React Native is one cause. Another is that they ship a tremendous amount of cross-platform c++ Office code, which is not small.
What I also find very annoying is that the Outlook app even embeds a whole browser (with GUI; is it Edge?). It makes me furious to be forced to use the embedded browser when opening links in emails instead of a preferred, already installed browser. I mean... what advantage has it to use an embedded browser? I lose my cookies/sessions, browser extensions, passwords, and the usability is worse than of my preferred browser.
What context are you seeing an embedded browser in?