The traditional 個 paired a person with 固 'solid'; the simplified 个 strips to essentials — a shape like a single bamboo tally standing upright. Ancient counting used bamboo sticks: one stick, one 个, one of anything.
个 is the Swiss Army measure word: when you don't know whether it's 只, 条, 张, or 本 — say 个. Native speakers do it too. 一个人 'one person' also means 'alone': 我一个人 carries whole moods. 个人 'individual' vs 大家 'everyone' frames China's oldest social equation.
One bamboo tally standing up — ONE unit of anything.
Mandarin has 150+ measure words, but corpus studies show 个 handles over half of all real usage — the ultimate default.