Why are all the Buddhas born in Indian subcontinent only?

Yes all buddhas were from india, that's a certain of world..In future also buddhas'll be born only in india..When a Bodhisathwa(buddha before enlightenment)looking for feasibility to be born, country and place are among major five factors, and always the country selected was india.. With the geographical positioning of india ,it has a special energy that is needed to make a buddha..As you mentioned in the same time greece,egypt...those countries also were developed but they didn't have that geographical factor which india had..And also otherwise buddha needed wise people who could understand the buddha serments, For that they needed to had spiritual and mental development, I think those western countries only had physical development more than mental development in that time..So buddha had to choose india to plant the seed of buddhism

It's Universal law. Which we cannot simply understand why.

We have to keep in mind the fact that general knowledge and education level of folks 2500 years ago was way different from modern time people. It was an age without internet and without any effective communication means. Geography wasn't a standardized formal academic discipline in schools, and majority of the population were un-educated folks. To them the concepts of Ancient Greece, Egypt, China, their cultures and languages were virtually non-existent. So if there was an ancient Buddha in China with a name "Chong", or Europe with a name "Kon", or Africa with a name "!Non", it would be much easier to be rendered as "John" by an American Buddha when He teaches His disciples in a subsequent period. The Dhamma would be much easier propagated to everyone once its ideas, concepts, and names were translated to the dialect local people back then could understand.

You surmised that the previous Buddhas were all born in the Indian subcontinent, simply because they had names that sounded Indian.

If you look at the excerpt from an academic paper below, certain Greek and Egyptian kings had Indian-sounding names in Emperor Ashoka's edicts, but this did not mean that they were Indian. Turamaya was Ptolemy, Maka or Maga was Magas, Alikasudara or Alikyasudala was Alexander. In fact, Greece was known as Yavana in Sanskrit (from Ionia). So, just because previous Buddhas had Indian-sounding names, it does not mean that they were Indian or that they resided in the Indian subcontinent. It only shows that they had Prakritized or Sanskritized names.

From the paper entitled "Antiochus, King of the Yavanas" by Jarl Charpentier :

As concerns Turamaya there can happily be no doubt. That it denotes one of the Ptolemies has been taken for granted ever since the days of Prinsep; and it seems quite obvious that none but Ptolemy II Philadelphus, whose long reign covered nearly four decenniums (285-247 B.C.) , would fit into the chronology of Asoka's reign." As for Maka or Maga there existed, no doubt, more than one princeling of the name of Magas; but there can be little doubt that we hare to do here with that Magas of Cyrene whose regnal years fall between c. 300-250 B.C. Already Buhler(3) remarked that Amtekina (G., K.) or Amtikini (Sh.) would rather render a Greek Avtigenes than Avtigenes. However, although we know of at least one Antigenes," he, for obvious reasons, cannot come in here. The old Antigonos who met his fate at Ipsus (301 B.C.) seems to be Out of the question; and thus there remains only his grandson, surnamed from the place of his birth Gonatas, whose reign extended between 276 and 239 B.C. Finally, Alikasudara (or Alikyasudala, K.) has long been taken to be Alexander of Epirus(5) who was the son of Pyrrhus and Antigone.

Just as fishes are found in water and monkeys are found on trees. Different races are found in different regions of the world. They differ in their skin color, language , necessities , dominating elements and philosophies. Out of this segregated reality Jesus , Ram , Buddhas are born. Nothing happens without reason. Jesus manifested because of the conditions around him. Ram or Krishna manifested because of the conditions around them. Similarly Buddha manifested because of the conditions around him. It is no surprise that Buddha came from Indian subcontinent because he came up with a theory of Anatta which was opposite to what was preached by Hindus. That insight required conditions created by Hindus. Similarly we can say Jesus came up his theory to resolve the issues faced by middle east people. Ram and Krishna also had a purpose which was relevant to their surroundings. No one was taught to be Buddha or Ram or Jesus, they were the product of Life and its environment. There should be no shame in saying that Indian subcontinent has been a place from where great sages and Buddhas came into being. If Buddha arises again then most probably he will arise in the East because East has the conditions for his growth.