The problem is that you're thinking in absolutes. That's not how life works. The driver doean't need to be a formal part of the hiring process that everyone rigorously goes through. There's variation and difference for every candidate. That's just because hiring is a human process.

In this case just knowing the driver well enough to ask them "Was that person nice to you?" when they drop a candidate off gets you another very useful signal when the response isn't neutral. A lot of the time it'll be "they were OK", but occasionally you'll get a positive "they treated me like a human being" or a negative "no, they were rude".

If you just get an "OK" then you're back to hiring as normal, without the extra data. If you get a positive or negative signal that can amplify what you see in the rest of the hiring process. For me, a tick in the "nice to people you don't have to be nice to" box is a good thing. I like to work with people who have that trait. If I can find a signal that indicates a person has it then I'm going to use it when I'm hiring.