I built a yogurt incubator, so I know a bit about how they work.
Yogurt incubators without temperature controls will have a preset temperature, and they'll heat the contents to this point with a thermostat or a temperature controller such as a modified PID controller. (Engineering: normal PID controllers are not suitable for cooking, since the heat-up phase will create a large integral error term, which will result in compensatory overheating.*) For a consumer product, it wouldn't make sense to hold an object at its current temperature, because that would assume the user set the initial temperature correctly. It would be a food safety issue to let the user implicitly set the temperature, without giving clear controls.
Next, it's not possible to hold an object at its current temperature if you don't know what that temperature is. Yogurt incubators often (usually?) allow you to use your own container, and there's no good (inexpensive) way for the device to read the liquid's temperature without touching it. If it used a typical thermometer (whether it's a thermistor or thermocouple), the milk would cool down by the time the device managed to get an accurate reading. On the other hand, if you are setting the temperature (arbitrarily heating to 43°C, for instance), all you need to do is measure the temperature of the plastic housing or the air--eventually, the milk will match this temperature.
*Note: my yogurt incubator does use a PID controller, but it's my product rather than a consumer product, so it only needs a certain amount of idiot-proofing. If I don't put overly hot or cold milk into it, it won't overcompensate and overshoot the desired temperature.