Cactbot can:
1. Solve mechanics that aren't visible on the client yet.
2. Solve mechanics with a less than 10ms reaction time.
3. Compile a series of mechanical queues taking place over a long time, and then provide a solution to a later mechanic, removing the need for anyone to remember anything
4. Make calculations based on Entity coordinates which tell you where to stand.
5. Can provide 8 separate callouts to 8 different people simultaneously.
6. Works out of the box with no practice or prog.
This is not at all comparable to a human shotcaller, who cannot do many of those things, and also has to spend prog time practicing to be able to do the ones they can. It fundamentally automates an aspect of fight execution that was not intended to be automated.
Cactbot makes callouts like "NE/NW are safe" or "Look SW". Many of the callouts it makes completely removes any necessity of needing to actually understand how 90% of the mechanic works, and it provides personalized callouts based on your characters specific circumstances. Those kind of shotcalls are really only seen in places like RichCOB, where 7 people knew what they were doing and could afford to make individual callouts for one member.
It doesn't just call out mechanics. It solves mechanics, and tells you what to do. It is not comparable to a human shot-caller. I don't really mind if people with rare disabilities use certain things for accessibility purposes, and there's an important difference between a trigger that just says "Three" when you get 3 above your head, versus a cactbot module that reads 3 different divebomb coordinates and then actively tells you where you need to stand because you have 3.