The manufacturers make money. They have lobbied for it. See that press release: https://finance.yahoo.com/technology/ai/articles/advanced-driver-distraction-warning-systems-071000637.html

Maybe local car manufacturers appreciate a barrier for importing foreign cars.

But maybe it is simply bad law-making. The law requires such a system. It does not require that it works. Many regulations in Europe are like that.

Partly that may be because industry likes it that way. If they were held accountable for deaths caused by their cars, they would have to do open-ended research. They might be held accountable for sneaky maneuvers. They might have to implement unpopular features, which the politicians would not like either cause voters. This way, they just tick boxes on a compliance checklist. It doesn’t matter if it works. That’s called “legal certainty”. Everyone says how important that is.

But I’m sure, in many cases it is simply incompetence. Neither the politicians nor the bureaucrats who write the laws know what they are doing. Clueless people often try to compensate by micromanaging. They don’t know the problems that causes. In fairness, the actual experts work for the industry that is being regulated. You can’t trust those guys. So there’s no reason to believe the experts when they say something is a bad idea. They say that about anything that costs them money.