A basic understanding of Newtonian mechanics reveals that some of our world is deterministic, and some basic understanding of quantum mechanics introduces randomness. Therefore, the scientific inquiry should be: Is our will deterministic or random? And it should not even be a debate. It is a scientific question that requires scientific discovery. It is not something that can be resolved through a debate. Yet, there are some people that we call philosophers, who are doing exactly that; they are debating, and they are not even asking the right question. They are asking whether will is deterministic or free. Is "Free" just a synonym for random? No. It is an entirely new construct they have fabricated.
So, what is Free Will? It is the idea that we decide our will. What? So, our thoughts decide our thoughts, and our will decides our will. It answers nothing, and it is an infinite regress. Yes, upon hearing this idea, that should be the reasonable reaction, but apparently, it is not. Instead, this “debate” has not only attracted the attention of the public but also of well-known scientists.
So, what is happening here? I will explain this using an analogy from religion. When a religious person asks themselves how the world was created, they answer, "God created it." Let us not ask the logical derivation to this conclusion and move on to the next logical question. If you have the curiosity to ask about the world's creation, logic dictates you should also ask, "How did God come into existence?" But the believer ignores this logical step. Logic was never employed during the whole thought process.
Analogous to this, a philosopher asks, "What decides our will?" They answer, "We decide it." Again, there was no logical derivation, but ignoring that, the next question must be: "What decides the will that decides our will?" This question generally never pops up in the mind of a philosopher. In both cases, the reasoning process never truly began.
The analogy proves that the reasoning ability of a philosopher is almost equal to that of a religious person. There are some differences, though. For a religious person, Determinism vs Free Will might mean “God vs Us.” And for the philosopher, it is “Physics vs Us.” Both are wrong because both assume "Us" or “God” are special entities capable of uncaused causes.
It is worth noting that if we look at the systemic level rather than the fundamental level, the brain does exhibit complexity. The output of "Brain System A" can act as the input for "Brain System B," or even create a feedback loop (feedback loops are also essential for consciousness). However, conflating fundamental mechanics and systemic networks to manufacture a debate still casts doubt on the reasoning ability of a philosopher.
So, this brings us back to the title: "Why did philosophers invent Free Will?" Honestly, I do not know. Perhaps some lack the capacity to reason. Or perhaps they understand perfectly well but realize there is something to be gained from this—likely popularity, and therefore, money.