system
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human
Why do manual and automatic transmissions use such wildly different gearing mechanisms?

I have been watching a bunch of videos on different transmission designs recently, and I am stuck on a seemingly straightforward question. Manual transmissions and automatic transmissions are designed very differently, with the latter using planetary gears and torque converter. My question is: why did automatics switch to this completely different gearing mechanism, vs. just automating the shifts in a manual transmission?

I am aware that clutchless and automated manual transmissions exist, but I am not aware of why the conventional automatic transmission design was adopted instead of them. Is this purely historical? (If so, what happened?) If not, what engineering advantages does a conventional automatic offer over an automated manual transmission? (The wikipedia page on automatic and automated manual transmissions did not give me a clear answer.)
gpt
Automatics were designed this way because all the gears are constantly in mesh, and are simple to control hydraulically without any electronics.  

Before the days of servo control, this was required.

An automatic transmission is *always* under a torque load, which means that you can't use a simple dog clutch + Synchronizer as you can in a manual.  

An Automatic transmission can be shifted while under load, so long as there isn't a locked link to the engine.  This is why an automatic has a torque converter between the transmission and the engine.

So in an automatic there can be a simple change in hydralic pressure to cause the transmission to shift.

In a manual, or automated manual you need to 

1. Disengae clutch
2. Change Gear, and allow the engine/transmission to match RPM via a dog clutch
3. Engage clutch

In a Hydraulic Automatic you need to

1. Change Gear by Locking/unlocking a gear set in the planetary.  As there is no direct mechanical connection between the transmission and the engine the RPM doesn't need to match.  The torque converter allows them to run at separate speeds.


Now with Servo Actuators, and Electronic control you can do whatever you like, and the driver is cost.