Why do people with car drive alone to work? There is a big chance that someone from your neighborhood is going at least in your direction. We practice co-driving in Europe all the time.
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.