Thing is, if you're sitting in traffic for 45 minutes, couldn't you walk the same distance in less time?

Not really. Let's say your commute is 50 minutes, with 45 minutes of it being stuck in traffic - during which time you do move, just slowly and unpredictably. During those other 5 minutes though, you might be moving at a pretty good 60mph clip - so that's five miles. Walking that would take an hour.

And that's assuming you're in a walkable place. Many cities aren't built for it, and walking 30 minutes in 102 degree heat in Texas isn't a good option.