Enemies do deal elemental damage and players did stack up resistance materia against them. Early on in 2.0, players would slot their accessories with Earth materia to better survive Titan's tableflip and earthquakes. However it was a stopgap measure as higher ilevels were much more effective in increasing survivability and pushing through phases than overmelding materia. And crafted gear never quite caught on the same way in raids or farm, hence the current situation.

Also, do not expect hard-coded elemental attributes like half-damage, null-damage, double-damage, and absorb to be included on bosses or for players. There's too much of a chance to exploit for that type of system.