It's all about Density Altitude - not what the above-sea-level altitude is. The airplane performs based on Density Altitude(DA), which is based on air pressure, which is a result of ambient temperature. For example, an airport at 1000 feet above sea level on a 100 degree day is actually about 3700 DA. Devner, at 5000 above sea level on a 100 degree day is 8500 DA!

Both LAX and JFK, being nearly level with the sea, have that benefit of not worrying about DA going much higher than 2000.