Oh yeah, it's totally understandable so no worries.
Hope it didn't look like I took offence or something. I was just scratching my head over the reason myself.
It's just a "path of least resistance" kind of thing.
Sure, ESRB and analogous entities around the globe have rules about age verification and warnings. Steam and app stores want to play nice with those organizations, so following those rules, perform some age verification. In the case of small/indie games released only through itch.io or Patreon, whose policies are either overly vague or overly strict-but-seldom-enforced, there's probably minimal risk to not including such a warning and verification in-game, but tagging the game as 18+ on the platform itself is important.
However, the time it takes to figure out whether your game needs or will need a silly and useless verification dialogue, is much greater than the amount of time it takes to implement one. It costs the dev almost no time, and costs the user almost no time to click through it, so devs put them in. If a dev were to be sued by some overprotective parent, then having the warning certainly wouldn't hurt the defense. It's a low risk, low cost, and low reward scenario, so there's no benefit to spending time or lawyer fees figuring out if it's actually necessary.