I've thought this many times, and I suspect it's somewhat complicated, engineering-wise — but solvable.

For one, detergent comes in at least three forms: Powder, liquid and those little plastic pouches. Powder would be pretty easy (but the dosage would be brand-specific) and liquid would be messy (flow rate would be a challenge).

The easiest way would be if all machines could accept a "standard pellet" which gets loaded in some kind of completely fool-proof way so the machine cannot mechanically choke on them, ever, or accidentally add too few/many to a load.

Same thing with dishwashers.

As for why you can't just close the door and walk away: Setting the program is an important step in washing clothes. Modern machines do have a single "start" button.

GE has both under the name "SmartDispense". They use a peristaltic pump to dispense liquid detergent. I owned the dishwasher for a few years and enjoyed the convenience.

In 20 years, I've owned 3 washing machines. I dealt with maybe 3 or 4 malfunctions over those 20 years, and each of them required only buying some spare part and installing it, or cleaning something inside.

Surely this kind of reliability is a good trade-off Vs having to pour some detergent for each wash?