You are constantly multi-tasking.

People manage to drive, navigate (i.e. watch for landmarks), and carry on conversations with passengers at the same time.

Men are particularly good at this. They can track a conversation (usually with a wife/SO) while watching TV/playing a computer game/reading a book, and manage to track the conversation--at least to the degree that they can make the appropriate single-syllable noises at the right times.

It is true that most people who claim to have this ability don't do it as well (or as pervasively) as they claim, and they are, in fact, time-slicing (even thinly), however there are people in other professions who do.

Watch a SWAT team (a good one) work a problem. Watch a good military officer carry out a mission. A trauma surgeon in an ER during a Mass Casualty event. These people are multitasking while working on extremely complex situations were a single mistake, missing a single word in a conversation (say dropping a "not") could get someone killed.

Watch an Air Traffic Controller at a busy airport, or the pilot he's talking to.

Yeah, people can multi-task, but not all of them, and not always well.

There probably is an upper bound on the number of tasks that one can work at a time, and (As another commenter alluded to) you've only got so much attention to go around, so as you dump more and more attention onto one task the others get "resource starved", but with work (like any other muscle) you can build up this ability to a certain degree.