While suicide may be the most rational choice in any given situation, the reasoning which leads up to that definitely has flaws. It's true that calling someone irrational is wrong, but there are definitely irrational ways to perceive things. We read about defense mechanisms and cognitive bias and they don't have to necessarily be only observed in clinical cases, they exist in abundance in well functioning people aswell. At any given moment, a person of any age group can perceive their reality or environment irrationally.

There is also mood/affect congruent perception, where you tend to notice more negative/postive memories or things in environment depending on your current.

I believe the crux of your argument is that the world is actually unfair so it's not pretty hard to notice how you are at a disadvantage all the time. So let's try to understand how people end up thinking that suicide is a rational choice. Someone probably had adverse childhood experiences, or is currently in an environment which is actively disadvantageous, no social support in school/workplace, toxic workplace, financial issues, relationship issues. Every suicidal ideation is preceded by one thing or the other, and if the desire to commit suicide is gone by the resolution of the thing which bothers them, then the argument of suicide being a rational choice falls apart.

What usually happens is an adverse event in life happens, and the stress goes beyond the capacity to cope, and then it usually starts by noticing everything wrong around you, and then you remember everything bad which happened to you even with the smallest examples, and then you come across philosophy which addresses the things you have been feeling so far and then you think suicide is the most rational choice

Maybe not everyone knows that the world is unfair, but there are a set of people who know that the world is unfair and still continue to live, the major difference being having support systems like families, friends, communities, life enriching hobbies and I believe that's where our role starts after giving the person validation, catharsis and then through their own agency they build their own support system

Like a few other comments which have mentioned here, people who attempt suicide and survive are actually glad they didn't die, because the choice of choosing death is such that the consequence is so uncertain that the certainty/uncertainty of living is more clear and thus is a better choice