Because it creates predictable incoming damage for players to need to handle, in addition to possibly avoiding attacks through movement, and allows for a predictable amount of outgoing damage to allow for eventual victory even with turtle builds (outside of hard timers).
Otherwise, it would create a situation where the skillcap is either too high for the average player to effectively survive combat due to taking too much damage - or a situation where skilled players can literally trivialize combat by essentially forsaking all defensive/healing abilities/items/gearing and just power down enemies.
It can serve quite a number of purposes, even if you don't like it.