why would nature be so cruel to remove your memories and wisdom reincarnation after reincarnation to make you step on the same stones again?

One of the Noble truths is acknowledging that pain or suffering exists rather than denying it so perhaps nature or reality is naturally cruel.

But it's not really reincarnation rather rebirth or re-existence in one of the five realms (hell, the animal realm, realm of ghosts, humans, devas).

Birth as a human is extremely rare the majority of humans most likely came from hellish worlds, the animal realm, or the realm of ghosts prior to this human existence and don't naturally automatically remember past existences it's only gained through developing one of the six higher knowledges (pubbe-nivāsanussati).

Christian Gnostics believed that it's because this world is controlled by Satan and only holy knowledge can give you salvation. Is karma (which is a religious belief) the only answer Buddhism has to this matter?

The concept of Satan might have originated in Buddhism as Mara the "Evil One" is a central figure in early Buddhism and pre-Buddhists religions seem to not have any Satan figure only post-Buddhist religions like Christianity and Islam which were obviously influenced by Buddhism since Ashoka sent Buddhist missionaries all around the world.

Kamma is one explanation but it's also explained in the suttas the nature of things is impermanent and subject to change.

why would nature be so cruel to remove your memories and wisdom reincarnation after reincarnation to make you step on the same stones again?

One of the Noble truths is acknowledging that pain or suffering exists rather than denying it so perhaps nature or reality is naturally cruel.

But it's not really reincarnation rather rebirth or re-existence in one of the five realms (hell, the animal realm, realm of ghosts, humans, devas).

Birth as a human is extremely rare the majority of humans most likely came from hellish worlds, the animal realm, or the realm of ghosts prior to this human existence (as implied in the Pansu suttas SN 56.102-113) and don't naturally automatically remember past existences it's only gained through developing one of the six higher knowledges (pubbe-nivāsanussati).

Christian Gnostics believed that it's because this world is controlled by Satan and only holy knowledge can give you salvation. Is karma (which is a religious belief) the only answer Buddhism has to this matter?

The concept of Satan might have originated in Buddhism as Mara the "Evil One" is a central figure in early Buddhism and pre-Buddhists religions seem to not have any Satan figure only post-Buddhist religions like Christianity and Islam which were obviously influenced by Buddhism since Ashoka sent Buddhist missionaries all around the world.

Kamma is one explanation but it's also explained in the suttas the nature of things is impermanent and subject to change.