The truth is that some car manufacturers actually request it to be somewhat fool proof, but very few.

It costs hardware to run and cost to develop/test, and car manufacturing is all about price.

But also:

Today the algorithm knows the 3D geometry of your head, and knows that it does not makes sense with a flat face. But it is more important to get it to work good on everyone than to catch foolers.

Disclaimer: i'm one of all the developers behind camera monitoring.