I don’t want to deal with other people when I am in the car. I like the solitude. Some days I may be working from home, some days I may go in late and/or stay late. I don’t want the responsibility of being a means of transportation for another adult.

In my experience (US, SF Bay Area/Seattle) - people do this for a few reasons:

1. The coordination and time overhead of sharing their commute with another person is not worth the marginal benefits - HOV lanes are not present in most of the area to make a difference in commute time and gas prices are low enough that splitting a drive isn't worth the savings.

2. Their commute also involves other errands (shopping, going to the gym, dropping off or picking up children from school/daycare) that are inconvenient to do with another person in the car.

In summary, it's convenient to drive alone and you are not penalized sufficiently for doing so.

We call it carpooling, and it's not that uncommon, I've done it. Some highways have special "High Occupancy Vehicle" lanes to encourage it.

May I point to this corresponding mode of transport for car-less people:

http://slug-lines.com/

Because it's better to not rely on anybody else, mostly.