Because I forgot it. To be precise, the 'this' keyword would only be needed in cases where the input variable and the local variable have the same name, and by using the 'this' keyword one can distinguish the local variable from the one passed as a parameter.

void setWeight(double age){ this.age = age; }

In which the assignment of the value of age is given to the local variable this.age. In the examples, I have reported there is no such problem because the local variable and the one passed as a parameter do not have the same name, so you can remove 'this' in all the code, and everything would still work correctly.