Explain what you mean by "objectively better"? Your response makes it sound like you don't know the difference and are doing it because everybody else does it. It also makes it sounds like you don't understand the difference between open source software and free software. Both are free licenses, open source is just one part of it.

The main difference is that GPL3 is a copyleft license, whereas MIT is not. Meaning that legally there is nothing in the license preventing a company from taking your code and using it for their purposes without having to contribute to improve the code.