You're talking out of your ass. Maintaining the exact temperature without a control system for a significant period of time without interacting with it regularly is hard.
You're talking bullshit. These types of control systems have been created for hundreds of years. It is not hard. Water has an extremely high heat capacity so keeping it at a particular temperature is pretty trivial, especially if you insulate the container well.
It is not a hard problem to solve well for a couple of tens of dollars with current tech. If you bought a system that doesn't work well then you were duped by an amateur wannabe engineer that doesn't know what they're doing.
A sous vide pan is just a temperature sensor, a heating element and some device to reads the sensor and switches the heater on and off.