why does my oil pressure go so low?

The oil pressure gauge in your car is electrically operated, mechanical oil pressure gauges tend to be more accurate. Oil pressure will be highest in the morning when the oil is cold and is thickest, As the oil warms up it gets "thinner" that is natural. At idle the oil pressure on many engines may be about 12psi and at 2500 to 4000 RPM the oil pressure is probably about 50 psi. If in doubt measure the oil pressure using a "know good" mechanical oil pressure gauge.