- Because of mechanical constraints and cost
- The interface is optimized for experienced users
The interface for a manual transmission is somewhat complex because it's a historical design driven by technological limitations. The shifter in most cars with a stick is still a mechanical linkage pushing around bits of steel inside a very complex set of gears. The abstraction leaks, in sometimes annoying but also useful ways, and determines where and how the stick shifter can be set up, unless you want to go to a very expensive system.
It's also tradition, and car UIs move very slowly. The "H" pattern has been honed over the years, including the spacing of the notches, the angle between them, the length of the throw, how hard or easy it is to push the stick around, and so forth. Likewise the engagement throw of the clutch pedal is something that car manufacturers engineer very precisely (for cars where this is a selling point).
Finally, the interface is optimzied for experienced, rather than novice, users--as most of us will be experienced users of our own vehicle, as well as experienced users of cars in general. Once learned, the feel of the stick and the pedal give the driver a lot of information about the state of the car's engine and wheels and the interaction between them, information that is lacking in an automatic. Having driven both extensively, it's clear that it is much harder to understand what is going on with your tractive surfaces in an automatic.