Besides preference for food there's also preference for sociability. If people prefer spending time eating with others, this could motivate them to not eat at home or in their car but instead in a restaurant. But fast food restaurants, especially with automation, is a less social experience than fine dining or eating in a pub. So maybe fast food fits a 'medium' level preference for sociability, while eating at home/in cars fits a low level of sociability.
What could drive preference for sociability?
- The number of singles is increasing and will make up a very substantial share of population by 2050 at least in Western countries (I'm less familiar with the situation in other parts of the world). A 'proper' restaurant (served at the table, no single-person or shared tables available, substantial waiting time for food) can be a very awkward experience for a single person eating alone. This trend could stimulate demand for fast food, and especially automated fast food. Or it could stimulate demand for eating at home and in your car at the cost of fast food.
- In some countries there seems to be a trend of (subgroups within) younger generations being less outgoing, like the otakus in Japan. A few days ago YouGov reported based on a survey in the US that millennials have fewer friends than earlier generations, with the most important (self-reported) reason that they are shy. If there's a trend towards more shyness and fewer friendships going towards 2050, this too could support a lower preference for social eating and a higher preference for either (automated) fast food or eating at home/in cars.
But you could also turn it on its head: if people are more lonely, they could actually have a higher demand for social experiences to counteract this. But that implies getting out of your comfort zone, which people aren't usually very fond of.
A scenario: by 2050 half of the population of most developed nations is single, and eats nearly all meals alone. They eat in their self-driving car with drone delivery (like in starspawn0's post) or food pick up at mobility hubs. If they eat in a restaurant, they prefer a fully automated experience in which food is ordered, served and paid for without any human interaction, and people eat at one-person booths with high quality video or VR entertainment. A small "social elite" looks down on this behavior and tries to eat as many meals as possible in slow-food, authentic restaurants or pubs. The fast food restaurant as we know it today (some anonymity, but also some human interaction and a limited level of automation) has disappeared.