Actually, you can. But you are required to use the scroll button on your mouse. Granted, it could use some more obvious means like buttons or a bar on the side as well. And watch out when you go over a line with lots of vehicles because the scroll button will also scroll that line of vehicles between left and right.
the best example of a mouse without a scrollwheel (or a difficult to use one) can be on a laptop. The fact that it is not obvious that the list is scrollable is an oversight from the devs who apperantly assumed anyone would try their scrollwheel on it.