Unfamiliar/unrelatable

I believe a term "emotionally unavailable" is often used to describe an avoidant.

I'd say the reason is that, at least in part, in order to recognise it in such specific terms, one needs to have both the desire and the ability to see the underlying causes behind any such behaviour in a relationship. Unless otherwise motivated, most people won't look beyond a surface level, or are incapable of doing so.

I'm sure there are many people who just see their partner as either moody, difficult, confused, or temperamental, without knowing why, and so, if and when those relationships fail they just put it down to that, without understanding the causes. In fact, if you look at relationship forums, it's possible to see many cases of relationships that fail for what we might now see as avoidance issues, which the affected party is blissfully unaware of.

Speaking as someone who sits more on the secure side, the day to day issues were less extreme, as a secure partner probably does less to trigger those extremes. As such, when a break comes, it might be because of something else, such as an argument, which might mask the underlying causes, and so you tend to take it at face value, shrug, accept it, and move on.

In fact, that was exactly the case in my recent break up, and ordinarily I would have put it down to that, and not given it much second thought. That was even knowing that my ex had abandonment issues, which she'd expressed but, like most people, I was unaware of just how deeply such issues run, and the internal conflicts they produce. It was only after the split that these things became apparent, as her subsequent behaviours didn't match up.

My getting angry and shouting, which was the "cause" of the break up, was never once brought up in any subsequent conversation. The initial anger on her part was never about that, and was directed only at me causing something "good" to end, and even that passed in less than a day. After that it was all good feelings, and an apparent desire to reconcile, but with her underlying fears now being expressed as the only obstacle. It was only the fact that she initiated contact after the break up that allowed these things, and other apparent inconsistencies, to become apparent; which, along with my desire to reconcile, prompted me to dig deeper. Had she not, or had she remained angry and unresponsive, then I'd have put it solely down to the initial argument, and left it at that.

So, depending on the circumstances, desires, ability, and motivations of one or both partners, any number of reasons could be assumed for such behaviours or break ups, before one might consider any kind of attachment issues or disorders.

In fact, the reverse is also true, as I've also seen examples where some immediately jump to the conclusion of such issues, when faced with relationship difficulties, where it's often apparent that it's merely a case of either lost attraction, or natural insecurity, and the subsequent actions of both partners have led to a cause and effect that anyone could choose to interpret as avoidant if they so wished.

As I've said in other posts, all people have a tolerance level to either rejection or being smothered and, if someone is pushed to their limit in either direction, they will act in an avoidant manner. An avoidant person just has much narrower tolerances, and thus more extreme reactions.

Something seems true about these points. In the book attached it starts off listing some very specific scenarios about relationship troubles and talks about how generally they all seem so different and so much the result of individual circumstances, tastes, and personalities as well as their specific combination but that taking an attachment theory lens they all have the same root problem, it's the particular feelings, symptoms, and hardships that are the result of these previously attributable variables.
Also, as humans we seem to have a great need for closure and I've done some reading on psychology research around breakups and one of the key stages in recovery is narrative construction. We need to create our own explanation of events to explain why things happened The way they did to move forward. This is often where we discount our own contributions to the problem and it is probably similarly comfortable to blame external circumstances or aspects of our partners and ourselves deemed unnecessary to change. Attributing events to such deep seated needs would probably require more insight and awareness than most people would like or are even capable of.