I find the advice regarding food a little mysterious, to be honest.

The guiding principle must be to minimise the risk of harming your unborn baby.

But if that's the case, then why aren't pregnant women advised to stop driving/travelling in cars or jay-walking (to cite obvious examples of a "risky" activities that we do every day)? The likelihood of having a car accident and injuring yourself is much higher than contracting listeria, pregnant or otherwise.

Listeria is an easy target because it's amenable to a controlled medical study, and avoiding certain types of food doesn't appear to infringe too much on what we think of as normal life. I'd be interested (for example) to see the relative risk to my baby of contracting listeria from eating soft cheese vs. sitting in traffic for two hours per day, or working with heavy-duty cleaning chemicals. Even if the last two were found to be more risky (although I imagine conducting a controlled medical study of them would be quite hard), I suspect most people would ignore them because it's impractical for most women to stop working or driving the whole time that they are pregnant.

Thinking through all the potential consequences of all the things we do every single day, then it seems obvious that a 100% risk-free pregnancy is an impossible dream. Ultimately, it's up to each of us to assess those risks and to decide which risks we are prepared to take.