Here's an answer in the style of Vicky Pollard...

Archaic Greek heavy infantry could well be described as "heavy skirmishers". Warriors are often shown carrying two spears, which must either both be javelins or include one javelin and one thrusting spear. In addition, even heavily armoured warriors are sometimes shown wielding a bow. In the relatively fluid fighting style of this time, neither heavy nor light infantry was separately marshalled, and ranged and close combat would have gone on between loosely organised battle lines simultaneously.

However, the heavy infantry type that the Greeks seem to settle on at the end of this period was a creature dedicated to close combat. He was armed only with a thrusting spear and sword; he carried a heavy double-grip shield and took his place in a large homogenous formation that was meant to charge directly at the enemy and decide the battle by hand-to-hand fighting.

There are a couple of reasons why this shift occurred. Part of it must be the fact that a swift charge into melee prevented missile combat from having much of an effect before the proximity of the enemy forced both sides to focus on their immediate self-defence. This was key to the Greek victory over the Persians. Of course, in any combat situation other than pitched battle, the hoplite was vulnerable because of his specialisation, but he still had his uses as a brute force weapon that could really only be stopped by men who fought in a similar way.

Another part, though, was that this fighting style acted as a great equaliser between the upper classes and the common man. Only those with the leisure to train could become good heavy skirmishers; these men had formed the core of Archaic armies, fighting as small bands of heavily armoured warriors who had the skill to use many different kinds of weapon. But towards the end of the Archaic period, more and more Greeks began to arm themselves as heavy infantry, and this broader section of the population could not afford to take the time out to master the art of skirmishing warfare. The resulting heavy infantry formation was therefore adjusted to the lowest common denominator. While it took time and effort to master the use of the javelin, the bow, or the sling, the Greeks believed any idiot could pick up a spear and shield and give a decent account of himself in close combat. This illusion was shattered in the face of more professional troops (like mercenaries), but such troops were rare on Greek battlefields, and the large untrained citizen levies of Greek city-states could probably not be more effective in battle than as a mass of angry men running at the enemy to stab him in the face. They therefore adopted a fighting style that relied more on raw courage than skill.

This is confirmed by the fact that the mass of Greek citizens who could not afford heavy armour, and were therefore forced to fight as light infantry, rarely achieved very much in battle. These people were not trained in the use of their haphazard weaponry, and could only really shine in very particular circumstances. The very effective light-armed that we hear about in the sources were often relatively small bands of specialist mercenaries, hired for the unique skills that their regional fighting styles had taught them.

These specialists left the close combat to the hoplites; they themselves practised a fighting style that focused on avoiding the hand-to-hand encounter altogether. As long as both hoplites and specialist light infantry were present, and the hoplites were mainly expected to engage in melee, there was not much incentive for either hoplites to develop ranged combat skills or for light infantry to increase their staying power. There were various ways for these troop types to work together. Rather than reequipping some hoplites as archers, they could, for instance, station archers and other missile troops within a hollow square of hoplites, which allowed the two troop types to combine their strengths.

The Greeks also realised that the best defence against light infantry was simply to field cavalry forces. Hoplites were never supposed to fight alone; whenever they did, it was because someone had fucked up. Any lightly armed skirmishing force would think twice about attacking a hoplite formation if any horsemen were about, as they could ride down the light infantry with impunity.

In short, the tactical system developed by the Greeks in the Classical period relied on the combination of different units with different roles. They were intended to work together in varying ways depending on the situation they were in. There was therefore no need to develop a sort of "universal warrior" who could face any conceivable threat, and if this warrior would reduce the effectiveness of hoplites in melee (since his weapons would make him unsuitable for it), he would actually harm rather than enhance the fighting methods of the Greeks.

All that said, the armies of the Hellenistic kingdoms did develop heavy skirmishers of various kinds to form a link between their even more specialised heavy infantry (the pikemen) and the light troops and cavalry that guarded the flanks of the phalanx. This is probably the nature of the Macedonian hypaspistai ("shield bearers") as well as the new infantry type called thureophoroi (after the Celtic shield type known as the thureos) that spread through the Greek world in the 3rd century BC. And of course Roman legionaries, with their throwing spears, are effectively heavy skirmishers until they engage in close combat.