Pre-Socratic (Greek philosophy before and contemporaneous to Socrates), Eastern (from the little I know about it), and much of Western philosophy are rooted in arbitrary metaphysical claims, according to Kant. The Pre-Socratics were particularly concerned with determining whether the world was one shifting reality (the One), two dueling forces, combinations of a few elements, or many atoms mixed into different mixtures. They struggled to answer how change was possible, whether the One merely changes shape, or if beings come in to and out of existence.
These philosophical problems have taken new forms throughout Western history. Christians, in particular, have used God to explain many of these questions. Some of them have become irrelevant, but Hume challenged philosophers to reckon with the fact that they have no access to these kinds of metaphysical explanations. We experience change in the world, but we do not experience the cause. We don’t even know if cause and effect is a real thing.
Kant accepted the challenge, agreeing that we don’t have access to arbitrary metaphysical descriptions of causes, but argued that humans necessarily interpret the world through the lens of cause and effect. We know that cause and effect happens, in a sense, because we experience certain events (A) that are always followed a specific different event (B), and never in reverse.
Admittedly, sometimes cause and effects isn’t instantaneous, such as the effect of cooking a pie in a heated oven for 45 minutes, but we are still justified in calling the heat the cause of the pie becoming baked because it is a repeatable event that can be historically situated.
Since Kant refuses to make claims about the “think in itself” he is not claiming that cause and effect is the nature of being (reality, the thing in itself), but that it is “real” in the sense that it is a necessary attribute of our experience of the thing in itself, whatever the thing in itself is.
P.S. Remember that in addition to this, Kant is also probably defending Isaac Newton’s casual laws as objectively true natural laws.