Why do we know about maximum 30-40 Prophets whereas we know that the world had seen 124,000 Prophets since its inception. Even Quran is silent about rest of the Prophets?
Second, an automatic question of why all Prophets came in an around a particular geographical area, I mean Arab peninsula. Why not in China, Canada or South America?
I believe that there is no authentic information about the number of all the divine messengers. The Qur'an only names a few.
As for the question that why all the Prophets mentioned in the Holy Qur'an were of Semitic origin.
First of all I would like to assert that it would not be very accurate to say that all the Prophets and Messengers of Allah were of Semitic origin appearing in Arabia. Prophets and Messengers before Noah are an obvious exception.
Though we can say that the available religious information regarding the places and nations to whom the Almighty sent Prophets and Messengers leads to the conclusion that the Almighty has selected the Peninsular Arabia and adjacent regions for the purpose of deliverance of Divine message. A chain of Prophets has been sent to establish a religious tradition to manifest the truth regarding the Hereafter in the very world. After the advent of the Prophet Abraham (sws) the institution of the Prophethood seems to be transferred to his progeny. These people after being trained by the Almighty through Prophet were raised to the status of being witness of the truth before the world (Shu`ada al annaas). They were made a symbol of the final Judgment to be taken place on the Hereafter in the very world. Whenever they remained righteous they were blessed in the very world and their opponents were surely destroyed. If, however, they abandoned the true path their matter was not put to the Hereafter rather they were punished in the very world. After the Israelites failed to discharg the duty to bear witness against the nations and for the truth the Almighty raised the Ishmaelite, another branch of the Abraham's progeny. This happened after the advent of the Prophet Muhammad (sws). Both still enjoy the status but the Israelites have to confirm the prophethood of Muhammad (sws) and stand by the truth in order to be at the right position. Both the nations dwelt in Arabian Peninsula and the adjacent areas. This is further corroborated by the fact that none other than Abrahamic religions claims to be of divine origin. Still we cannot rule out possibility of messengers being sent to any other people of the world, as the divine sources do not deny this.