Light bulbs are used in physics problems because they are simpler to model and analyze than LEDs. The filament in a light bulb is a resistive element, which means that it has a resistance that varies with temperature. This makes it easier to model mathematically than an LED, which is a more complex electronic device. Additionally, light bulbs have been around for much longer than LEDs, so they have been used in physics problems for a longer period of time. However, LEDs are becoming more commonly used in physics problems as they become more widespread in everyday use.