Mostly because the rest of 5e is built around an assumption of relative balance.

Adventures in modern D&D tend to consist of a series of more-or-less balanced encounters, usually combat, that will tax but usually not kill the player characters. If you tune it to be too easy, that makes for a boring session, or one where the DM runs out of content because the set piece encounter didn’t last as long as it should have. If it’s too difficult, you might have PCs die in a way that doesn’t match expectations. If most of the time combat encounters are supposed to be balanced, and a player has invested in their character’s backstory, and there’s clearly an arc they’re supposed to follow to the end, it sucks to have them be eaten by feral dogs.

“The DM can fix it” is always true, but a cop-out. If players avoid a set-piece encounter in 5e, it feels like they’re avoiding the whole dang adventure. And while XP doesn’t have to come from combat, that’s the bulk of it, and the most clearly supported by the rules.

And other systems just don’t have the same problem. Narrative games, like Blades in the Dark, have characters face consequences but not die unless it would be narratively satisfying. Other games just aren’t built on the assumption of balanced encounters, so it doesn’t throw a wrench into things if players get an unfair advantage, or bypass an encounter altogether, or just plain run away. And something like PF2e, which is in the modern D&D model, does have a functional balancing system.

A functional balancing system also doesn’t really have the problem of constant, perfect balance. D&D’s CR system will let you design encounters that are Easy, Medium, Hard, or Deadly, and PF2e’s Threat levels include Trivial, Low, Moderate, Severe, and Extreme. It’s just that one works better than the other.

Obviously all of this is “fixable” by the DM, but still, that puts a lot of work on the DM just to make the game work as intended.