The Macedonians (Greek: Makedones, Makedones) were an ancient tribe that lived on the alluvial plain around the rivers Haliacmon and lower Axios in the northeastern part of mainland Greece. Essentially an ancient Greek people, they gradually expanded from their homeland along the Haliacmon valley on the northern edge of the Greek world, absorbing or driving out neighbouring non-Greek tribes, primarily Thracian and Illyrian. They spoke Ancient Macedonian, a language closely related to Ancient Greek, perhaps a dialect, although the prestige language of the region was at first Attic and then Koine Greek. Their religious beliefs mirrored those of other Greeks, following the main deities of the Greek pantheon, although the Macedonians continued Archaic burial practices that had ceased in other parts of Greece after the 6th century BC.

Both Strabo and Thucydides said that Emathia and Pieria were mostly occupied by Thracians (Pieres, Paeonians) and Bottiaeans, as well as some Illyrian and Epirote tribes. Herodotus states that the Bryges were cohabitants with the Macedonians before their mass migration to Anatolia. If a group of ethnically definable Macedonian tribes were living in the Pierian highlands prior to their expansion, the first conquest was of the Pierian piedmont and coastal plain, including Vergina. The tribes may have launched their expansion from a base near Mount Bermion, according to Herodotus. Thucydides describes the Macedonian expansion specifically as a process of conquest led by the Argeads:  But the country along the sea which is now called Macedonia, was first acquired and made a kingdom by Alexander [I], father of Perdiccas [II] and his forefathers, who were originally Temenidae from Argos. They defeated and expelled from Pieria the Pierians ... and also expelled the Bottiaeans from Bottiaea ... they acquired as well a narrow strip of Paeonia extending along the Axios river from the interior to Pella and the sea. Beyond the Axios they possess the territory as far as the Strymon called Mygdonia, having driven out the Edoni. Moreover, they expelled from the district now called Eordaea the Eordi ... The Macedonians also made themselves rulers of certain places ... namely Anthemus, Grestonia, and a large part of Macedonia proper.  Thucydides's account gives a geographical overview of Macedonian possessions at the time of Alexander I's rule. To reconstruct a chronology of the expansion by Alexander I's predecessors is more difficult, but generally, three stages have been proposed from Thucydides' reading. The initial and most important conquest was of Pieria and Bottiaea, including the locations of Pydna and Dium. The second stage consolidated rule in Pieria and Bottiaea, captured Methone and Pella, and extended rule over Eordaea and Almopia. According to Hammond, the third stage occurred after 550 BC, when the Macedonians gained control over Mygdonia, Edonis, lower Paeonia, Bisaltia and Crestonia. However, the second stage might have occurred as late as 520 BC; and the third stage probably did not occur until after 479 BC, when the Macedonians capitalized on the weakened Paeonian state after the Persian withdrawal from Macedon and the rest of their mainland European territories. Whatever the case, Thucydides' account of the Macedonian state describes its accumulated territorial extent by the rule of Perdiccas II, Alexander I's son. Hammond has said that the early stages of Macedonian expansion were militaristic, subduing or expunging populations from a large and varied area. Pastoralism and highland living could not support a very concentrated settlement density, forcing pastoralist tribes to search for more arable lowlands suitable for agriculture.Answer this question using a quote from the following article:

Why did they do this?
Pastoralism and highland living could not support a very concentrated settlement density,