Military techniques proven effective tend to spread pretty quickly, especially if they fit into the socioeconomic setups of their recipients. Before the Macedonian phalanx the Greco-Mesopotamian hoplite phalanx had been "the thing" and was used by just about every group that fancied itself as "civilised" around the Med (and more to the point had the sort of agricultural society it was suited for).
Well, the Macedonian pike phalanx trounced it soundly, and duly became the norm. The Romans went straight from hoplites to maniples and didn't bother with pikes along the way, but they were pretty much the lone exception - and their javelin-toting swordsmen did have some issues with the pike-hedges.
The German "phalanx" in the game is probably stretching it a bit. I'm quite willing to accept the Teutonic spearmen could have fought in the sort of close, spear-armed shieldwall that would remain the norm well until Middle Ages, but if that one rates as phalanx then the Triarii should too... Well, write it down to artistic license and some creative interpretation of Caesar.
Compare the popularity of the phalanx - of either form - in the Antiquity to the way pikes became all the rage during Late Middle Ages and the Renaissance following the Swiss example, or the spread of firearms anywhere once introduced. Imitation, as they say, is the most honest form of praise.
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