Because we are not clever enough to come up with them yet. Then again, there MAY be things like physical constants whose values are 'givens' and cannot be derived from more fundamental considerations. Until we have a self-consistent theory of quantum gravity, we will probably not know for certain just what constants are truly fundamental or not. It seems every logical system we have devised has to begin somewhere, with some 'givens' to jumpstart the process of deduction. The speed of light can be derived from Maxwell's equations as the square root of the ratio of the electric and magnetic permitivities of free space. But to understand why THOSE constants have THEIR values only carries our discussion one level deeper and it once again dissolves into 'the values are what they are' with no further explanation.