My local Toyota dealer, when they change the oil, sets the engine light to come on at 3,000 miles, even with synthetic oil. I don't' understand why & asked them stop. They continue to do it. Why?

The light is preset at 5,000 miles or 6 months from factory and cannot be changed. The reason the light comes on every 6 months is because tire rotations and inspections are recommended from Toyota every 6 months. If the vehicle develops a small leak or issue, you don't want it to go a year unnoticed. It's easy to reset anyway.