Radar detectors work by detecting radar that's being shot up ahead of you and giving you time to adjust your speed accordingly before it's your turn to be clocked.
Radar jammers will actively jam the police radar gun, preventing it from getting a reading.
The thing is that unlike radar detectors, radar jammers are massively illegal. I'm talking huge financial penalties and prison time. Additionally, the radar jammers that were produced years ago don't work well against modern radar guns and fall out of tune over time. That combined with the legal issues means that no one has made anything new to jam radar for obvious reasons.
So radar detectors are cool, but radar jammers are not.