There's a bunch of other private frameworks (SafariCore.framework, SafariFoundation.framework, SafariPlatformSupport.framework, SafariShared.framework, SafariSharedUI.framework, SafariSwift.framework) as well. I haven't checked but I assume it's similar on iOS.
Maintaining a browser is expensive and with the way Apple implemented their obligations, it's not worth it for existing browser companies to ship their own engines (and have twice or more the maintenance).
