Because who wants to have control over what they do while in combat, right? That's probably the LAST place I would want to dictate my own actions rather than leave it to chance.
The problem with eliminating RNG in combat is that it would boil down to rock, paper, scissors mechanics. Chance, fate (whatever one wants to call it) plays a role in combat. Some way of simulating probability is used. Dice rolls are simply easy to use. Other systems have used playing cards.
Certain role playing systems that are diceless have either simple mechanics for combat (rock, paper, scissors) or very complex ways of resolving combat to the point of tedium (for many players).
The player can control the actions of the protagonist and party, but should have to guess at what the enemy may or may not do based on gather intelligence. RNG also allows games to simulate weather conditions and other variables beyond the party's control.