First of all, people aren’t uniformly good at splitting pills. Dose deviation is common among pill-splitters and that’s an issue for drugs that need to be maintained in a narrow range. Worse, many pills are in extended release formulations that break down when split. Splitting a time-released pill in half can cause an overdose. The list goes on: cutting all your pills in half before you need them could cause them to become ineffective, people often forget to cut them and take a double dose, etc.