why cant i use iphone x for digital car key function

Well, you’re lucky. You just have to buy a new phone. I would have to buy a new BMW, but I have my eye on a Tesla 😏

Although currently I unlock my car by touching the door handle, never have to touch the keyfob. So having to take out a phone would be a step backwards.

Because the iPhone X does not support the feature; only the XR/XS and later do. And currently BMW is the only manufacturer that supports it.