In the older editions, like the ones you’re talking about, casters had serious downsides. Between being very fragile, spells being interrupteable, and sometimes having different XP amounts, casters were kinda ‘glass cannons’, and needed a martial frontline.
In 3.5 and 5e, casters have had these harsh downsides decreased or removed, while not otherwise losing power. They are more or less strictly better than martials, in the sense they can do 90%+ of what martials can do better than they can do it, while also doing several other things. And the few things martials do do better, it’s by slight degrees.
It’s not just that casters are powerful, it’s that they’re powerful and flexible, able to be top tier in several different roles at the same time, and can change what roles they cover by resting and swapping spells.
Whereas martials can sometimes build to be top tier in one role, but they’re largely locked into that one role, or can build to be okay in several roles (and be outclassed by casters in all of them).