This isn't just a problem exclusive to MMOs, but something that has been showing up with increasing frequency in even gachas and regular games like Xenoblade Chronicles and Granblue Fantasy Relink. Rather than looking to see an attack incoming in the game world like you would seeing a giant raise his arm in Shadow of the Colossus, we now have brightly colored circles on the ground telling you where you need to step out of. Why is this happening? Are devs becoming too incompetent or lazy to animate enemy attacks that hit where they appear they should?
It is at least partly because being at the maximum distance from the center of a fireball does the same amount of damage as being in the middle. A crisply defined edge is required if the damage is consequential.
There also isn't any concept of different damage types in games these days. So it's not like the warlock is immune to shadow damage and different players have to look out for different mechanics. Developers have fallen back on information overload and reaction time as the core design over any sort of decisions or resource management.
Easier to design encounters with them + the addition of cast bars. It would be a whole different world if those things didn't exist, or were limited to expressive animations (diff animation for diff spell). Personally I think it reduces the immersion drastically. Not that having 40ppl hit a bajillion times with their daggers and maces the base of a 50ft wind elemental doesn't do that too, but anyway.