Why doesn't the outer electron on the potassium atom experience the full charge of the protons in the nucleus (+19)?

There are 18 electrons shielding it from the nucleus

Explanation:

The shielding effect is the reduction in the effective nuclear charge on an electron, due to repulsion by the inner electrons in the atom.

Potassium has 19 electrons, 18 of which are in the inner shells of the atom. These 18 electrons "shield" the outermost electron from feeling the effect of the nuclear charge. The magnitude of effective nuclear charge felt by an electron as a result of screening by other inner electrons is given by Slater's rules.