/u/Iphikrates made a pretty good response to this in At what point in time was the Macedonian Phalanx completely outclassed on the battlefield? so I'll take a few parts of that to summarize and add a bit of my own things in hopes that someone will make a more targeted attempt to answer your question.

The concept of obsolescence isn't as simple as you might think it is. Even during the height of the Macedonian phalanx, Alexander's invasion of Persia, the phalanx itself was only one specialized piece of a larger army. Think of the battle of Issus, where Alexander defeated Darius III, the battle was won because of a cavalry charge that broke Darius' formation and caused him to flee to prevent being killed himself (Diodorus Siculus 17.34), which caused his lines to collapse. It wasn't accomplished because the Macedonian phalanx formation was somehow superior to Darius's less advanced (but still Greek heavy infantry) core. Phalanx made up less than half of Alexander's army in that battle.

In a modern anecdote, consider that many modern American wars are fought with bombs and missiles, but it doesn't mean that the automatic rifle is "obsolete", it's just one specialized facet of a military. The phalanx, in that sense never really became obsolete until the entire concept of hand-to-hand fighting became obsolete. It was just one specialized part of an army that could be wielded with effectiveness in the situations it was designed for.

The other posters have focused on the Macedonian "pike phalanx" so far. But in case you meant to refer to the classical "hoplite phalanx" with one handed spears and large shields, then you might also be interested in this excellent post by u/iphikrates, where he discusses whether this formation really was unique to the ancient Greeks.

For more on Greek hoplites you could also try looking through his profile here: https://www.reddit.com/r/AskHistorians/wiki/profiles/iphikrates

In short, guns.

For a bit of background in this thread /u/Itsalrightwithme and /u/XenophontheAthenian combine to compare and contrast Renaissance Swiss pike blocks with Greek phalanxes.

Short answer, a long spear remained highly viable well into the early modern age, especially when mixed with some sort of firearm. These pike & shot formations dominated the early modern battlefield. They required a fair amount of training and coordination. But variants of a spear and shield formation remained common for millennia. In fact without professional pikemen the early gunners would have been terribly vulnerable.

But then as /u/vonadler discusses in this thread the pike portion of pike & shot was eventually replaced by more shot and bayonets. As guns became more effective, it eventually became practical to phase out the spears. It was eventually better to have more guns who could act as lousy spearman in a pinch than to have dedicate like units who couldn't act as musketmen at all.