@hammerlane That's a good point about a teen not being lazy, but being overwhelmed by the thought of driving. A teen also might not being willing to admit that they're afraid, or even aware of the root cause of their fear.
In the first couple drives, many of my teen driver ed students express trepidation about driving on the freeway, often, for the same reasons as your wife. But by the time we head out onto the freeway on Drive 5, they've built up enough confidence that it's usually an easy transition to higher speeds.
That's also a good point also about driving on arterials and highways as actually being more dangerous that driving on freeways, since, as you say, the variety of speeds, directions, and road features make the driving experience a lot more unpredictable, even though it's a slower speeds.
Glad to hear your wife has gained confidence on the freeway!