It's sadly not very marketable as it happens roughly 15-20 times a year in the US and everyone thinks it won't happen to them. When it was designed as a third party product it was a car-seat based weight detector rather than something that was a direct extension of the car.

I think a little indicator on your car with lights for each seat that is occupied where occupied is done by weight and is able to ignore the weight of a car seat might be a useful portion of an electronic dashboard. Still dashboard real estate is scarce and I'm not sure what indicator we'd be willing to remove to have it. On the whole it still doesn't surprise me that car manufacturers haven't done it.