It's those single serving kind of friends, ala the plane scene in fight club.

Temporary human mutual benefit alliances. You ride along, notice a fellow driver is placing safety over convenience. So, when car homie needs a slot to change lanes, you adjust your speed to hook them up. Next time, maybe they give you a headlight flick when they notice something coming from behind that is placing convenience over safety.

It's just humans being decent to each other on the road, but it fulfills a pretty deep human need for connection in one the worst settings for human connection.

Then, when they or you take an exit finally, you wave, give a honk, and move on.

But I'm telling you, and you find someone that you can drive like that with, it's awesome. On a really long drive, you can sync up and make better time with less stress. If you get a small group going? Awwwww yiiiiissssss.

I've had the good fortune of hooking up in a group a handful of times over the years. One time, going up to Ohio, a group formed and when one guy signaled and went into a rest stop, we all followed him in and just made small talk until he was done. Went like that all through until right over the Ohio border, where we had to split off for our individual destinations. We'd switch off positions, give hand signals back down the line when something was wonky ahead.

Never exchanged names, never talked about anything serious. But those were some great road friends

No, but you do start to consciously personify the drivers – contrasting the extreme depersonifying that driving encourages (see: "carbrain"). It's all wild, baseless speculation, and you know that, but it's fun to construct a fake traveling buddy in your head based on tidbits and wonder if maybe they're doing the same to you.

(Edit: I will note that I see this as kind of an "orphan-crushing machine" type of wholesome, showing how insanely isolating car travel is through how the human mind will cling onto the tiniest scraps of closeness with another person.)

For sure. Surely women do this too, right?
It's a driver thing, right?

This exactly it. Same approach to driving, speed, passing, being in the correct lane, lots of miles to cover.