Earlier quoted context omitted.
It's frustrating because someone is taking your safety buffer as their opportunity to travel faster. And it results in you having to travel slower and slower to maintain the gap that is constantly consumed, tragedy of the commons style, by opportunists.
Slow down a bit to create another buffer. You can even do this before they have merged, as part of the bit where you allow them to safely merge. I think if you reflect a bit you'll find you are being the same kind of person as them, if you are getting angry that you have to slow down and give up space for someone else. I understand some people can be aggressive though, that can be frustrating regardless of the outcom…
We're not talking about where they're changing lanes to take the next exit. We're talking about where your lane happens to be moving faster, so they merge in front of you in an unsafe way to take advantage of that and just stay there. Why should you be expected to give them space, as you suggest? How is that fair, that they should get to their destination faster instead of you? Do you not see how that's going to rightfully make someone angry? When they should be waiting for a safe space to open up, rather than forcing you to slow down to create one?