It’s a multiplayer game. You are supposed to see other player’s attack animations. It would be extremely strange to see the enemies falling down around you without them. Many builds have AoE damage attacks. The red and orange colours of Sorc or Necros might just be a bit too “noticeable”.
Having to manage transparency for those animations would probably have a significant impact on performance. There are already problems for some of them.
I play Fireball Sorc, usually take an Elixir of Advantage to increase my attack speed when doing Helltides. Stand in the middle of the altars and just spawn fireballs all around. Sometimes a Blood Surge Necro joins me, sometime another Fireball Sorc and it get even better.