@masukomi Classic Traveller has even less character improvement. It expects to spit out some characters at age 22 with Pistol-1, Medic-0, and that's you for the rest of your adventuring career. And, frankly, that's not great to play; everything interesting about your character you have to invent yourself, and the rules won't help you. You might as well just make up a story.
Call of Cthulhu has the virtue of less randomness in CG, so you may not have been able to choose your stats, but you could at least choose your background and job.
I think CoC shows that you can have very little character improvement, certainly not enough to compensate for the progressive loss of sanity, and keep things fun.
An "iconic" character like Doc Savage or the Shadow doesn't improve, indeed comes fresh out of the box for each adventure. That's fine if that's the sort of narrative you want to engage in, but that isn't all narratives.
Of course a big part of the problem is the blighting effect of D&D's baby version of the Hero's Journey being the default way to think about RPGs.