Why did Greek Equipment get lighter?

[Poulp']

I wonder if all peltast armies in EB (or at least 66% peltasts , 33% elite shock hoplites -NO phalanxes, they cannot support the peltasts properly-,cav support,archers[wait, are archers even necessary anymore with a peltast dominated army?]) would be a feasible idea....peltasts seem to be even more flexible than the roman legions but better in terms of cost upkeep, and requisite infrastructure. *imagines 8 or 9 units of fully armor/weapon upgraded peltasts*

I used to run a peltast-heavy army of Numidian Javelineers playing as Numidia (it was on vanilla, mind you)
The results were terrifying for the enemy, it's like chasing a mirage in the desert.
And their formation was dense enough to slip through any gaps and shoot/charge from behind.

It requires something to take care of enemy cav (jav cav) and shooters (light cav) and a mini anvil behind which the skirmishers can fall back (desert infantry)
It requires a lot of micromanagement too.

[Leão magno]

This happens until our days, foot soldiers are always getting more specialized and lighter in equipment and capable to deliver the same amount of destruction as the earlier heavier units