Uhh, short answer: it's for convenience, not security.
--Easily obtaining passwords from your browser is a feature, not a bug. support.mozilla.org even provides steps that are readily googled:
"For the passwords you need to copy logins.json and key3.db. For the bookmarks you can either use places.sqlite or restore a compressed .jsonlz4 JSON backup from the bookmarkbackups folder. "
Uhh, short answer: it's for convenience, not security.
--Easily obtaining passwords from your browser is a feature, not a bug. support.mozilla.org even provides steps that are readily googled: "For the passwords you need to copy logins.json and key3.db. For the bookmarks you can either use places.sqlite or restore a compressed .jsonlz4 JSON backup from the bookmarkbackups folder. "