They are at this point pretty much as "classic" as dwarves and elves in fantasy settings so why do you guys think they did not include them?
Because Goblins and Orcs are lazy. The idea of an evil society devoted to conquest is beyond unrealistic. Furthermore, it leads to unrealistic conflicts. A small group of chosen warriors defend civilization against the illiterate, uneducated, unwashed masses, and other fantasy barf. This problem is aggravated when writers conclude that there must either be perpetual interspecies conflict, or that the solution is a rigid caste society. The only time they are done well is when they're given a distinct culture with conflicts that make sense, and PE already has enough heavy trope lifting to do. Each traditional fantasy element added has the significant risk of detracting from verisimilitude and makes the task of writing significantly harder.
Bear in mind, the same criticism holds true of traditional elves; they're the inverse of orcs and just as unbelievable. Josh et. al seem to have had a significantly hard time making them interesting and relevant for that reason. But I would argue that elves are more core fantasy than orcs, because stories of elves and faeries played a significantly larger role in English canon than orcs.
Dwarves don't have the same problem. They're different, but very obviously driven by human motivations.
Interesting point of view that is quite logical too. But then, it all depends on how you design these races. For example in the Warcraft universe orcs are quite balanced, although in the Middle Earth one it's not: they're just the "generic" evil guys. But I really think that there are ways, if you want so, to integrate, at least orcs (maybe halflings too?), in a realistic and satisfying way.
I would like to reflect on something that you said though: "The idea of an evil society devoted to conquest is beyond unrealistic." I don't necessarily agree. I'm pretty sure for example that the modern day image of the orc civilisation that we have (since tolkien let's say) has taken its roots in the underlying imagery european civilizations had in the middle ages of invaders from the east, basically turks and mongols, and one could actually say that you're description of a "evil society devoted to conquest" corresponds pretty well to what the turkic nomadic invaders represented in the european collective consciousness and therefore is not unrealistic (without the "evil" part maybe), so it's not unrealistic to have conquering nomadic civilizations and there are ways to include orcish civilizations that could seem realistic and logical. (btw no racism here, I'm turkish..)
And lastly, I don't necessarily think you have to flesh out an entire civilization to include some races. You could very well have an orcish civilization in a region far away, not in the game, and have some rare orcish mercenaries employed here and there and the orc race available in character creation.
Anyway I don't necessarily criticize the design decision of not including them, I personally believe they could have been included while also maybe giving them a particular, new, fresh, PoE touch to them, if the designers wanted to do that. it's true that halflings have been somewhat put into the game in the form of orlans though. I would have liked orcs to be included only in character creation for example, that would have been cool with me and the game wouldn't have lost that much of personality..