I'm sure there are studies of driver distraction when talking to someone who is in the car with them, but I've never seen those studies publicized or discussed. Common sense suggests that talking to someone sitting next to you, which includes normal human behaviors like looking at them for visual cues, has to be far more dangerous than hands-free cell calling. It's possible initiating a hands-free call is briefly more distracting than an in-car conversation, but that's a UI design issue.
City busses in Seattle have "don't talk to the driver while the bus is in motion" signs... for a good reason: they don't want you to distract the driver. So, why aren't there laws restricting conversations with people in your car? Beyond the practical impossibility of enforcing such a law, I suspect this has to do with technophobia, where "tech" is defined as something that seems new, rather than the tech we are already embedded in and used to. Cars themselves have gotten to the point of being assumed infrastructure, which is why we accept the general carnage on our roadways without question, while being up in arms about an increase in accidents due to mobile usage.
Another example of this kind of technophobia (or maybe technophilia in that it's what people want to hear about) is the publicity about studies of bacteria on cell phones. Sure, cell phones are rife with bacteria. But what about other things you have in your pocket, touch regularly, and would never think to clean, like keys and wallets? Keys have to be at least as bacteria-infested as your phone, and they haven't killed us off yet (except indirectly, through car crashes and sitting too much at the office). But I haven't seen any studies of general pocket-stuff bacteria publicized here or anywhere else. I suspect they exist, but where's the angst? Like the bacteria themselves, keys have been with us for a while now, so no angst.
Other commenters have noted that laws against cell phone use and texting do little to reduce usage. The advent of always-on remote connectivity to people and information is the killer app for self-driving cars. Young people instinctively realize driving cars get in the way of this connection, so they have stopped wanting to drive cars. Making a self-driving car is super hard/slow, but not much harder/slower than getting auto makers to do decent telematics UI design.