Why does supermarket basil die so quickly?

Because a supermarket basil pot is not one plant. It is dozens of seedlings sown densely into a small volume of compost, grown fast under glass, and intended to be harvested whole within a week or two. It was never produced to live on a windowsill.

That density is the core problem. Twenty or thirty plants competing for the same tiny rootball exhaust the compost and its nutrients within days, and there is no room for any of them to develop a proper root system. They have also been grown in a controlled greenhouse at high light and steady warmth, then moved to a dim kitchen with draughts and fluctuating temperature — a severe check for a plant that is already stressed.

Watering habits finish it. People often water little and often, keeping the surface damp and the lower compost either bone dry or waterlogged. Basil hates both.

If you want it to last, divide it. Knock the whole mass out of the pot, gently pull it apart into three or four clumps of a few seedlings each, and pot each clump into its own container of fresh peat-free compost, one size up. Water thoroughly, let the surface dry between waterings, and put it in the brightest spot you have — basil wants direct sun, not a shaded shelf.

Harvest by pinching out the growing tip above a pair of leaves rather than stripping individual leaves from the bottom. That forces the plant to branch and keeps it productive.

Even then, expect weeks rather than years. If you want a long-lived plant, a single basil grown from seed or bought as a garden-centre specimen will outperform a supermarket pot every time.