It may be easier to imagine someone trying to derive mathematics all by themselves, since it's less abstract. It's not that they won't come up with anything, it's that everything that even a genius can come up with in their lifetime will be something that the whole of humanity has long since come up with, chewed over, simplified, had a rebellion against, had a counter-rebellion against the rebellion, and ultimately packaged it up in a highly efficient manner into a textbook with cross-references to all sorts of angles on it and dozens of elaborations. You can't possible get through all this stuff all on your own.

The problem is less clear in philosophy than mathematics, but it's still there. It's really easy on your own terms to come up with some idea that the collective intelligence has already revealed to be fatally flawed in some undeniable manner, or at the very least, has very powerful arguments against it that an individual may never consider. The ideas that have survived decades, centuries, and even millenia against the collective weight of humanity assaulting them are going to have a certain character that "something someone came up with last week" will lack.

(That said I am quite heterodox in one way, which is that I'm not a big believer in reading primary sources, at least routinely. Personally I think that a lot of the primary sources noticeably lack the refinement and polish added as humanity chews it over and processes it and I prefer mostly pulling from the result of the process, and not from the one person who happened to introduce a particular idea. Such a source may be interesting for other reasons, but not in my opinion for philosophy.)