Smart Audio Report on Smart Speaker Adoption in the U.S. – MacStories:

To me, this suggests that either owners of smart speakers aren't familiar with smartphone assistants (I doubt it) or that there is indeed something useful about stationary speakers that smartphones can't fully replicate.

Federico Viticci nails it here. As I have tried to articulate as well, there is a distinct difference between a smart speaker with a voice assistant and a smartphone with a voice assistant. If people like Alexa/Assistant and when asked, say they want those assistants in their cars first, and then their smartphones, one of two things is most likely true:

1. People don't know that their smartphone has a voice assistant built in, and if it's an Android phone running Nougat or newer, it's the same Google Assistant as your Google Home.
2. The use case between a phone and smart speaker are different and people do not find the current implementation on phones to be sufficient.

I would give people the benefit of the doubt and say that number 2 is the answer.