Why don't car companies like Chevy Ford ect. Have built in radar detectors?

What agreement is in place here?

Because most people don't want or need them.

Sure is funny though to watch guys get snagged because the cop turned on the radar at the last second when it is too late.

Congratulations dumbass… Instead of getting to the next traffic light a whole 15 seconds before me, you'll get there 15 min later and a whole lot poorer.

Not unlike built-in GPS units the technology (radar detection and maps for GPS) change fast and the update/upgrades are either impossible due to hardware/firmware limitations or prohibitively expensive over modular units

It is kind of like a radio in a car. If you want one, buy one and install it. Otherwise, there's not much demand for radar detectors as the police can tell you if you are speeding or being targeted by aliens.