Yes, active cancellation is simple in theory, hard to implement in practice.

In practice, it only works when either:

a) The noise is emanating from a specific "point" such as a loud machine of some kind, and you can mount a speaker very close to the loud machine and point the speaker exactly at the listener. And remember that the speaker must be able to match the timbre and volume of the loud machine to be able to do any good. For example, to quiet the sound of a diesel engine, you'd have to have PA-system size speakers and huge amounts of amplification.

b) The sensing microphone and the speaker can be very very close to the listener's ears. They have successfully implemented this in noise-cancelling headphones for years, but it's only useful because it's all self-contained in the headphones and it's right next to the ears.

I don't know of any other examples that work, does anyone have any other examples of active cancellation?

The problem with trying to do it in the cabin with car speakers is that you would have to do it with multiple microphones and then tune it for only one passenger's ears and it still wouldn't be perfect. And as soon as someone else got in the car, the noise would be worse for their seating position, you couldn't have a "universal" setting for multiple passengers.