I'unno, while true there also tends to be a fair bit you can do. Mostly base it off incursion, m'self, but a level 1 D&D character can do some pretty impressive stuff relative to a real life human. Also get flattened by a cat, but they can do it while having fancy tricks, if generally not even remotely powerful. Is the magic of death world housepets, I guess.
That said, weak to strong is just... standard. Freakishly standard, as in I'm pretty sure that's literally the trope name (or at least the tag on novelupdates :V). There's high falutin' literature dongles about why it slots into storytelling archetypes and blah blah blah, but it's mostly just a done thing. Contrast or somethin' makes the power high of later bits more noticeable, plus some of that hero saga thing what has a term for I can't remember. Folks are just used to it, used to working with it, and so on.
... that said, game wise it's probably half because they want progress but don't want mid/late game amount and quality of content in the beginning, too. Imagine if you had the normal RPG scaling, but your start was end-game and the finish was sweet zeus. Friggin' post macguffin punch wizard territory* at that point, with sky fingers exploding mountains and the protag beating people with literally a river and crap. Then think about having to make models and animate and code and etc., and realize your funding is not actually the entire US GDP.
Anyway, bunches of reasons it happens so much. Some good, some habitual, some economic. Not just RPGs that do it, it's varyingly standard in a lot of genres and storyboarding styles.
* Martial arts fantasy stuff, less colloquially. Think crouching tiger hidden dragon direction, except intermittent laser eyes and blowing up the occasional planet or whatever. Dunno how much gets officially localized but there's freakish amounts of amateur translation efforts for written stuff and even more freakish amounts of junk to translate so *shrugs*