You probably have plenty of novel ideas in early career, but you almost certainly lack the experience and the basic understanding of your field to develop them properly. Most people have exhausted their own ideas by mid-career. But that that point, they should have the skills and the experience to work on the ideas they come across.

(Looking back at my PhD, it's quite amusing how little did I understand. On the other hand, many of the choices I intuitively made turned out to have some value. But in some cases, understanding that properly took a decade of work by other people.)

Your PhD work is an apprenticeship, after which you are expected to work as a journeyman. The masterpiece that qualifies you for independent work as a tenured professor is often called habilitation. Many academic cultures don't have those, because the expectations are so situational that they don't want to formalize them.

In some ways, people doing research now have it way more difficult than people of the past. They have hundreds of years worth of research to study before they are on top of things and making an original contribution that stands out among the huge amount of research that already exists is really hard. If we want to keep PhD as a proof of meaningful work, then we ought to lengthen the graduate studies considerably. How about a 10 year PhD program, at the end of which you can really say you have mastered the field?