In my experience, the Greek-speaking world teaches Greek history as an interconnected progression, with ancient developing into medieval, and medieval to modern. Yet, my experience in a British, Anglophone university is that Ancient Greek history is taught completely separated from later periods of Greek history, giving the impression that the ancient Greeks happened, then disappeared, and then people calling themselves Greek re-appeared suddenly in the 19th century. On the other hand, at the same university you could learn about how the western Roman Empire developed into Anglo-Saxon England, or Frankish France, Lombard Italy, Vandal Spain etc. Is there a reason for this?
[this is my first ever question, so I apologise if I haven’t managed to adhere to all of the posting rules. If the question has issues, please feel free to suggest edits etc., and I can try again. Thank you]
This is called periodisation, and it is common in all disciplines. It is academia's natural tendency to seek edges or boundaries in the stream of historical events. Some push back against this tendency in modern scholarship. I personally think that some periodisation is necessary and warranted, while as a general practice it should not dictate terms. There was a dramatic shift in many facets during and after the Bronze Age collapse. It wasn't the same in all areas and it did not happen in a single day, but there is a boundary there. There were immediate world consequences to the death of Alexander in 323, and I think it is silly to argue that there were not. Likewise, the world did change after the Battle of Actium in 31 BCE.
In the case of Greece, what you are detecting is a very tangible fissure between the "Greece" of Antiquity and what came later. Most aspects of culture, religion, political thought, and philosophy which characterized the "Greece" of antiquity were gone in most of Greece by the 6th century CE. Some might argue they were gone long before, under a long Roman hegemony. Theodosius I outlawed "pagan" religious practices in Greece in 391/2 CE. The last Olympic Games were held in 393 CE. The old ways continued in some places, of course, especially out in the country, but the urbanized Greeks on the whole were now Christian. The great Neoplatonic Academy in Athens was shuttered in 529 CE (officially, anyway). These developments in themselves are major boundaries between Antiquity and Late Antiquity.
There were also hegemonic and territorial changes. From the 4th to 7th centuries CE, Greece suffered greatly under repeated threat and invasion of "barbarian" elements. Nominal control passed from western Roman to Byzantine interests, but neither could do much to stop the Goths, Huns, and later Slavs. Many of the jewels of Greek antiquity were pillaged during this period (Delphi, e.g., where the dedicatory offerings of the previous fifteen centuries were carried off or lost to the sea in over-laden ships).
After the sixth century, mainland Greece undergoes a sharp decline in economic power and urbanisation. Greece was now a stop-over in the historical record for much of the rest of the Mediterranean and larger world. Greece would be divided between controlling factions like Byzantines, Franks, Venetians, and Genoese in the Medieval period. It would be Venetian ships which would finally blow up the Parthenon, one of the last surviving major monuments of the Greek old world.