Very little applications strictly need a turing complete machine. However microcontrollers are ubiquitous because they are so flexible. One chip does the job of

Very little applications strictly need a turing complete machine. However microcontrollers are ubiquitous because they are so flexible. One chip does the job of many, and without needing to be specialised, allowing all applications to take advantage of the economies of scale in semiconductor manufacturing. (Also as pointed out here you drastically underestimate the complexity of a modern ECU)