why does supermarket basil always seem to die within a couple of weeks indoors?

Supermarket basil usually dies quickly because it is essentially a "forced" crop. Here is why it struggles to survive in a home environment:

• Overcrowding: Those small plastic pots contain dozens of individual seedlings packed together to create a lush, full appearance for the shelf. They are competing for the same limited soil, water, and nutrients, which eventually leads to them crowding each other out.
• The "Hothouse" Shock: Supermarket basil is often grown in high-humidity, controlled greenhouse environments with intense light. When you move it to a typical home, the air is usually much drier and the light levels are significantly lower, which causes the plant to go into shock.
• Root-Bound: Because they are grown quickly for sale, the root systems are often cramped inside very small containers. Once the plant outgrows this pot, it cannot absorb water or nutrients effectively.
• Light Deficiency: Basil is a sun-worshipper. Most indoor windows do not provide the 6–8 hours of strong, direct sunlight the plant needs to sustain its growth. Without enough light, it becomes "leggy" and weak.
• Improper Watering: Basil dislikes "wet feet." If the soil stays constantly soggy, the roots will rot. Conversely, if it dries out completely, the soft stems collapse quickly.

A quick tip to extend its life: If you buy one, try dividing the clump into 3 or 4 smaller pots with fresh potting soil immediately. This gives the roots room to breathe and reduces competition. Keep them in your sunniest window and only water when the top inch of soil feels dry to the touch.