I find there's an asymmetry between listening and speaking. It's fine for someone else to tell me about their week, but I can't reciprocate with the same level of detail.

I suspect when driving demands attention, it's easier to buffer incoming words to handle a second later, or to simply miss portions and recover.

In contrast, a buffer of outgoing words between mind and mouth is harder to manage, and people prefer not to fall silent in awkward socially-unacceptable ways, especially if the other person might not see what diverted you.