For the most obvious reason—that the kit won’t necessarily measure all harmful substances, including organic, inorganic, and microbial growths like certain algae and pathogenic microorganisms—relying on a kit to test water is naïve and ignorant.
The water might kill you over a period of time ranging from long to short depending on the amounts of toxins not covered by the kit, but you could easily obtain acceptable and very limited analytical results.
The recommendation is to have a public, chartered water analyst evaluate the water. They can advise you on which specific, harmful compounds to have analyzed. It is essential to specify the well’s precise location in relation to nearby companies, industrial plants, and animal farms.
Comprehensive water analyses will not be cheap to begin with, but they may increase your life expectancy. You can focus future analyses solely on such dangers once harmful substances have been tested and shown to be detectable and at safe levels (the water analyzer can advise you of them). Activated carbon can be used to entirely remove organic, and non-ionic substances from water if their concentrations are too high for safe ingestion.
If the levels of metal toxins are too high, using mixed ion-exchange resins will remove them, but ideally without removing vital minerals.