Or people whose attention trails off when you talk to them.

To be fair, this can happen a lot when you are simply not being interesting/engaging/concise/direct/confident enough. Or possibly just babbling on about things they really don't have any interest in.

There are a lot of different causes, but the general end result is the same. Their eyes go kinda vacant and far away and you can see that you don't have their attention anymore. Perhaps they are even subtly shifting away from you towards their intended destination too.

It's not that they're not listening, or are bad people or whatever, it's just that they saw where you were going with this ten minutes ago, but they don't want to be rude and interrupt you either, plus they're thinking about a million and one things of their own, etc..

I have learned to get my point across in two sentence soundbites

This is a good thing. Practice until you find the right mix of efficiency and small talk that works with him, I guess.

But the thing is, some people are more distant than others, but if you're encountering people that zone out on you frequently, the problem likely lies with you, I'm sorry to say.

People used to do that with me a lot when I was younger and had the ADHD and social anxiety less under control. I used to kinda slouch and slink away, not really meet people's eyes, stammer a bit and stumble around my main talking points out of nervousness, over-illustrate what I wanted to say, etc. You get the point.

Now I take my time and breathe before speaking, tell them exactly what they need to hear and not much else unless asked to elaborate, and square my shoulders and look them in the eye, no slouching either, I stand up to my full 6'.

tl;dr: It may not be them, it might be you