What are the disadvantages of tight coupling in object oriented programming?

what is the disadvantage of tight coupling other than you can write unit test code better in the case of loosely coupled classes?

I know with loose coupling you can write unit test cases and I know it suits the SOLID principle. but I didn't understand what will happen if I don't do that.

To me what is tight coupling is when you instantiate an object of the class directly without any factory class and without using the interface just directly creating the object of it.