Many people want privacy and control over the vehicle they're in, and will pay a premium for it. When you take a bus you're sharing it with random people, can't stop to take a break whenever you want, and can't go directly door to door.
Trains can be a superior alternative depending on what country you're in. In the U.S. we're far behind some other countries in high speed rail infrastructure, so over long distances trains are usually slower than driving.
If there is political will to fund the research needed and create the infrastructure for autonomous highway driving lanes, surely politicians in the US can find the incentives to fund the infrastructure needed for extra buses (including luxury lines with first class seating; for those willing to pay the premium) and train lines.
Edit: As an example of political will for the established and proven solution of mass public transit system, over the unproven non-existent solution of autonomous highway driving lanes. Look at the steam (pun intended) the idea of establishing high speed rail between Portland OR, and Vancouver BC is getting from the public and politicians alike.
Trains can be a superior alternative depending on what country you're in. In the U.S. we're far behind some other countries in high speed rail infrastructure, so over long distances trains are usually slower than driving.