It is wrong to say that the Romans relied so heavily or only on infantry. That was perhaps the case later on in the prime time of the empire with it's standing army and professional soldiers, fighting more or less against not so well organized "barbarian" foes. The Romans always used strong cavalry elements. Without it battles against the Punic or Hellenistic enemies were very dangerous.

Pike use in the middle ages was more aggressive than in antiquity. The phalanx was a perfect element to pin the enemy down and finish him off with other troops, especially cavalry. On it's own as in the later Hellenistic time it was difficult to decide a battle with an attacking phalanx alone. Ptolemy did it at Raphia, with the help of his elephants, but that was a seldom event. Against the Roman infantry the phalanx was effective as long as it kept formation. At Kynoskephalai this formation was shattered by Greek cavalry and the attack of the Roman elephants.