It used to be pretty common for computer-illiterate people to add a bunch of extra search bars and other weird hotbars, often of ill-repute, by accident. Maybe Apple just didn’t want to repeat that.
The status quo in fact encourages going back to the toolbar-infested days of yore. The only way to use a non-bribe-paying search engine in Safari is to install a browser extension with access to every webpage. There are standards that work just fine for switching search engines without any extensions or toolbars but Apple explicitly rejects those standards.