Armor in the late fifth century bc.

[hoplite14gr]

Olympia Museum for thigh guards and arm guards plus contemporary Etruscan statuetes.

From 700 Thespians 400 died at Delion

As for 400 B.C with much of Greece ravaged it meant that with your crops destroyed (including your linen) and scarcity of money you could not rearm easily.
If you were also in the loosing side and paying tribute things could be worse.
So in case of an emergency you went with what was available

And after walking several miles with both modern and ancient armor I assure you that if it comes to protection Mediterranean heat is just a minor inconvenience.

Also states who could afford sizable mercenary continents could transport them by water effectively and cheaply. No need for long marches.

And as a large number of mercenaries were either peltasts or coming from poorer districts (i.e. Arkadia) that never fielded many or very heavily armed hoplites the lack of armor is rational. Plus you stand to make more as lightly armed raider than getting killed in a stand up hoplite fight. A good unit of drilled heavy infantry would cost more that bunch of cutthroats armed javelins and knives.

Kind regards

Kind regards