That's, uh, not anywhere near a universal rule. That's just bad mods.
Great example, look at the spellpack mods for Skyrim. A couple of them are just adding back spells from older TES games and are fairly balanced (mostly due to not retaining certain spells and exploitable mechanics). A couple are well-balanced and spread across all the schools and levels of play, innovating both in mechanics and art design. A couple are god-mod packs that do absurdly broken shit.
Like, you're talking an incredibly narrow range of mods (added-content mods whose content deliberately and consistently outclasses vanilla content in similar roles) and then speaking as if it was symptomatic of a problem common to most or all mods. Added content mods aren't even the bread and butter for most games, they're little side dishes that you add on as an afterthought to stuff like combat system overhauls, crafting reworks, difficulty increases, texture packs, fan-expansions, &c.
Hell, look at the nuCOM 2 workshop. It's flooded with content addons, sure, but check the top-rated all time. The vast majority of the most popular and well-liked mods are QoL changes, expansions of incomplete vanilla content (like voices for all the nations), and a handful of well-balanced content packs (many of which are "balanced" in the sense that they're uniform cosmetics which fit with the vanilla tone). You don't even see all of the little bits of the Mass Effect conversion obsession until the second page, and I don't know how far back you'd need to look to find god-mod stuff.