Okay, first of all, that's not a real number. At its peak the Athenians may have had 400 trireme hulls in their docks, and if they were to find crews for all of them, that would have meant a navy of 80,000 men. But this never actually happened. The largest number of Athenian triremes ever used in a single battle was 200 (at Artemision). During the early years of the Peloponnesian War they would sometimes have slightly more than that in active service, but they wouldn't all be gathered together. The largest fleets they used in that war included only about 100 of their own triremes. The rest of the trireme hulls were there as a buffer against losses, to be equipped in emergencies, or simply to satisfy the popular wisdom that Athens should build more triremes every year, regardless of need or practicality (Demosthenes 22.8).

The problem with equipping larger fleets (or fielding larger armies for that matter) was not just manpower, but logistical and tactical management.

Logistically, very large armies put an enormous strain on subsistence economies; you were effectively moving a very large city into the territory of another city and using up local reserves to feed it. Large armies and fleets could not exist for a very long time in the ancient world unless they could draw on a well-organised system of pay and supply.

Now, Athens in the early years of the Peloponnesian War may have been able to fund such an army for a while, but that doesn't resolve the problem of tactical management. Simply put, the larger the army, the harder it is to control. For ancient battlefields this is a purely practical reality: when you field hoplites in the thousands your battle line is going to stretch for hundreds of meters. Since the Greeks had only the most rudimentary signals and no collective training, you won't have any means of communicating orders to the other end of the line. Naval battles were even worse because ships needed a lot of room to operate; naval battle lines would stretch for kilometers. The Athenians tended to achieve better results with smaller forces that could be managed directly.

The same problem would affect the huge mass of light-armed troops you're imagining here. Light infantry needs room to operate; it needs to be able to dash in and out of range, and to be able to retreat quickly when heavier troops try to close with them. It takes training and experience to do this well, which rowers armed ad hoc do not have. They also need a lot of time to do damage, since untrained missile troops cannot generally inflict many casualties quickly. If there are very many of them, they will be fighting more or less independently and making their own decisions, since they will be beyond the sight and control of any commander.

In other words, the more light infantry you have, the harder it would get to find a battlefield large enough and a battle long enough to allow them to function. If they get trapped, they will be helpless against hoplites and cavalry. If the enemy can force a decision (for instance by routing the Athenian hoplite force or killing its commanders) your light-armed troops are unlikely to keep fighting. So it would take a lot of planning and manipulation of the enemy to even allow this army to do its thing, and even then the outcome is far from certain.

Because of these problems, the Greeks rarely relied on large swarms of light infantry in their battles. They certainly brought them along for campaigns, to provide missile cover, confront their counterpart in the enemy army, and to ravage the countryside. These troops had a great many tactical and strategic uses. But pitched battle was the environment in which they were least effective. Battles hinged on hoplites and cavalry because those were the troops that could hold ground or disperse a phalanx without taking all day to do it.

This is not to say, of course, that the Athenians never tried to arm naval crews as light infantry. The relevant example of the battle of Sphakteria in 425 BC: thousands of Athenian rowers landed alongside hundreds of hoplites and specialist light troops on a small island garrisoned by a few hundred isolated Spartan hoplites. The Athenian general made his light-armed mass more effective by dividing them up into units of 200, which could be more tightly managed and could respond to circumstances quickly, some retreating before the Spartan advance while others attacked their flanks and rear. Even then - outnumbering an unsupported enemy more than 10:1 - they had to fight and manoeuvre all day and only managed to force the Spartans to surrender by getting behind their final position. It was a showcase of what these troops could do, but only in the most favourable circumstances. The Athenians were extatic with their victory, but no one would have suggested they try the same on a larger scale against the entire Spartan army.