Philosophy is not like software engineering. There's a lot more reading that needs to be done before you can make a contribution. A lot of it progresses by just reading the works of the established masters, getting ideas, and publishing your thoughts about what they had to say. You don't start from scratch. You build on what is already there. Addition by addition the field progresses and removes bad ideas or clarifie…
If philosophy is reasoning in its purest form, why so much reference to other work? Shouldn't it be like math where we logically conclude something and it is true regardless of what others think? If not, then it seems a less pure form of reasoning than math.
If you are not aware of all that accompanying discussion, you are likely to think you have come up with something profound when you actually haven't. This is why r/im14andthisisdeep/ is a thing. A lot of teens think they have an insight despite the fact that what they thought is neither profound nor novel.
Also, a lot of philosophical reading helps set up frameworks or theoretical scaffolding that can serve as a layer of abstraction that you can build upon instead of everyone rediscovering the same ground principles.