Design philosophy in the West seems to be more inclined to passive armour than to ERA, atleast with current MBT's, and for example Leo2A5/6 or Merkava 4 do carry additional armor modules, but in form of NERA or other layered beasties (much similar to the Russian T-55M / AM2 armor kit), from various reasons which I don't pretend I know
If you look say at Israel, you'll find that: Centurions and M60!s received ERA at first in Lebanon, but relatively soon the idea was dropped and Cents were passed out while Mag'ach's got passive arrays of increasing quality, finishing probably with Batash and Merkava 4.
So the decision why Greek tanks don't have ERA is prolly that a) the obsolete ones are well... obsolete so the cost would be higher than the benefit and the new ones have adequate protection already so again the investment would be moot.