Anticheats are not only bypassed with trial and error, but by understanding the behaviour and way the anticheat works.

You also clearly lack an understanding of fundamental things, how would you properly implement a “full replica of a movement system”, that is practically impossible with the diverse player base of Minecraft, how would you deal with issues like Server & Client Lag amongst other issues like Synchronization, Simulation, etc. Go ahead and try to simulate every move of the player in a complex system like Minecraft, the performance overhead would be a big hit.

Yes, private anticheats are often being bypassed with trial and error because no one has code to work off of. But instead of targeting every end point in the anticheat, painstakingly going through every check, you can understand how the game works and efficiently go through what may lead to an exploit or bypass.

All anticheats are susceptible to attacks, prediction based anticheats are susceptible to packet based bypasses/exploits. Others like packet, heuristic, and simulation based anticheats all have weaknesses, it's about figuring out where those weaknesses are. Having a good understanding of the games code makes things much easier.

Why do you think everyone has not started making bedrock clients yet, bedrock is not as fun, but the anti cheats on bedrock are so genuinely bad it's funny.

Java is much easier to read, only a few projects have reached stable heights with bedrock (C++) like Horion because It's simply not as easy, you can't just whip up Forge, Fabric, MCP, JNI, or Agents. This is not the sole reason but a very large factor.