Why constants is not integrated in integration factor ?
Are you talking about why the constant does not change in an integrated equation?In the resulting equation, the constant is just the coefficient of a variable to the first power.Or are you asking about the "+ C" you add after it's integrated?When derived, a constant w/o a variable becomes zero. When integrating you are finding the equation that your original equation was derived from.
The reason constants is not integrated in integration factor is for simplicity reason. Adding or subtracting any constant will give you another antiderivative and therefore, one has to ignore it.