cap's endings in the mcu have always been terrible.
in cap1, cap either is a terrible pilot, or he deliberately divebombs the superbomber into the arctic with himself onboard.
in cap 2, cap deliberately drops his guard and lets bucky, a supersoldier serum powered amnesiac with a robot arm, punch him in the face until he falls unconscious.
almost the entirety of cap 3 is a fucking mess.
and now we see in endgame that cap apparently ages just like a normal person, unless he did something really fucky in his timeline (which is the best possible explanation, since none of the others paint him in a very good light).
i don't know why the mcu couldn't be better written. apparently tons of stuff gets cut that would have established character, motivation and the basic logic of this universe, and since some of these cuts are only seconds long, i'm not sure why these cuts are being made. aou was particularly bad about this. i hope this will change but in all honesty, i haven't watched an mcu film since ant man (besides gotg 2, which is admittedly an improvement upon the first), and with the way they utterly fucked spider-man, i'm probably not going to watch another of these films until they get around to filming f4 or the xmen -and i will admit that it will be purely because of nostalgia and curiosity. i do not have an expectation of quality or comic accuracy since ant man pretty much turned the mcu from a quasi realistic action movie property into a disney cartoon (with writing far below the norm for most kids cartoons).