The other comments have effectively lambasted the manufacturers for their incompotence and foolishness in putting these on the same system, so I'll skip that bit.
The practical reason is that the big central touchscreen is a convenient place to put stuff. On an old truck, you selected 4WD by moving a big transfer case lever, which indicated you were in 4WD by its position. No computer to get in the way. Slightly newer trucks have physical buttons with lights that connect to electrical solenoids, often directly. Tractors have brake pedals for each wheel, but old trucks don't have a similar braking force distribution system - you got locking diffs or dragged all the brakes and hoped it helped. With a touchscreen and a computer in place, it's easy to make compelling sales pitches for putting complex features there instead of on expensive, complicated physical controls.
Complicated on a per-unit basis, that is - I understand that a computer has far more moving parts than any plastic-and-wires assembly, but when it works it's easier to draw buttons and graphics than to make mechanical actuators.
Whether or not the processing happens on the same CPU or communication happens on the same physical bus doesn't matter that much when you have the ability to make selections that affect the engine and drivetrain on the main display.
The practical reason is that the big central touchscreen is a convenient place to put stuff. On an old truck, you selected 4WD by moving a big transfer case lever, which indicated you were in 4WD by its position. No computer to get in the way. Slightly newer trucks have physical buttons with lights that connect to electrical solenoids, often directly. Tractors have brake pedals for each wheel, but old trucks don't have a similar braking force distribution system - you got locking diffs or dragged all the brakes and hoped it helped. With a touchscreen and a computer in place, it's easy to make compelling sales pitches for putting complex features there instead of on expensive, complicated physical controls.
Complicated on a per-unit basis, that is - I understand that a computer has far more moving parts than any plastic-and-wires assembly, but when it works it's easier to draw buttons and graphics than to make mechanical actuators.
Whether or not the processing happens on the same CPU or communication happens on the same physical bus doesn't matter that much when you have the ability to make selections that affect the engine and drivetrain on the main display.