They can be deterministic but I think you might be confused about the kind of game

In RTS games like 0AD or AoE you don't just send a single huge unit to attack and wait for the result, you send many tens of individual units near enemy units, then the "battle" goes on in real time and you can micromanage units to influence the outcome. You can't just simulate it on the server because the server can't simulate the thought process of the players

AoE2 battles are all about the micromanagement. Last minute splitting the onslaught of trash units against your opponents treb micro shot can change everything.

Kind of old but lots of micro tactics per unit here: https://youtu.be/hjUgisPD_C4?si=F-UvzDOTsWRZhZSq