Basil will bolt prematurely for various reasons but it's generally the result of some form of stress. Having too little space in a container is one. Were yours in a container? In the ground? Sown in situ? Transplanted? Watered sufficiently?

The basil were planted in outdoor garden, good soil, watered well, and close to full sun. The plants were purchased as small plants from local garden center.

Were they root-bound i.e. was the pot they came in packed with roots?

No, the seedlings were healthy. I had 4 plants, all the same.

I pinch my basil about every 2-3 days and remove them from seeding/bolting. Below this plant in a raised bed ended up 4 feet tall.

The plants tried to bolt again and again, even after frequent pinching.

I just keep pinching and pinching