I'd say it's customer expectation. The slogan in the Chrome website is: Now more simple, secure, and faster than ever - with help from Google built‑in [0]
So, if I install Chrome I expect Google search to be built-in, and I am okay with that because it was clearly stated on the product page.
Now, when I install Office, which is not a browser to begin with, I don't expect it to change my default search provider.