Problem is: They cant spam 3000 troops at once in the screen. There will not be machine powerful enough to handle that.
So the only way to implement big armies, is by waves.
The game spams a max of 1024 agents if i remember well, and then it need to wait until garbage collector kill some agents (lets say, 200) to instantiate another 200s and keep the 1024 limit.
You could say "But total war games spawns 20,000 troops" Yes it does, but they act like a block guided by a single AI component, different to Mount and Blade games, where each agent on screen have his own "brain" and "decision threes".
They could probably forcing the engine to max spawn 3000 agents maybe, but again, even a i9 9900k with 2x 2080 ti super and 32gb ram will struggle to get 40 fps. Now, how many people have that rig? Maybe 3 of every 10 clients.