I explained the reason to block off Shaaloani earlier: going there early would force an incorrect and unfavorable reading of Koana's story, by showing certain visual elements before they're ready. Additionally, it'd force addressing the zone transition to Heritage Found, which has both an even higher out-of-universe barrier ('zone doesn't exist yet'), and a considerably weaker in-universe barrier (the train is... actually completely fine, and if it wasn't the path is walkable if long; there is actually no barrier).
As for Amh Araeng, I can make a couple cases; it's situated after you've learned a lot about the First so the existence of Nabaath Araeng can build on that preexisting discussion. In addition, we actually spend pretty much all of Shadowbringers on a clock imposed by Vauthry and Ran'jit; the entire sequencing of zones basically comes from them chasing us and us having to make on-the-fly decisions. Going to Amh Araeng before you're meant to is just absurdly dangerous and breaks the narrative flow of Shadowbringers for absolutely no good reason.
But really, there's a much more base reason that applies more than any narrative: as Exmo pointed out, it's a waste of your time. FFXIV's developers want you to get to the fun parts, and a zone is more fun once you have narrative context, some quests to fill out both main and side, and the assurance that you're at the right level of strength to challenge it all (most people don't actually enjoy walking through an overlevelled hellzone for no reward; if you do, may I suggest Eureka). So, they prevent people from unwittingly making the wrong decision by sealing off a zone before it's ready; this is mostly to prevent people who genuinely might make the mistake of walking into a high-level zone without knowing it, rather than someone like you who's doing it deliberately, but... no, it's also to ensure you don't deliberately waste your time in the corner of a high-level zone doing god-knows-what and then complaining becuase you're three zone transitions away from where the actual content is.
DPZ2 also made a pretty good point on a more functional end a couple pages ago: the more space you give people to explore in without gates that require actually playing the game, the worse RMT bot farms get. That's more Heavensward's gates doing their work than Shadowbringers and Dawntrail's, though.