It's because the Infinite subtitle whatever added onto the Halo brand caters to the largest casual audience who, upon seeing the trailer, suddenly believes that all their "classic artstyle" dream fetishes will come true, and that our lord and savior 343 has finally saved us from the damnation they themselves made for us. /s
Seriously though, I hate the title. It's dumb, and really generic. Halo 5: Guardians is a good title. Bad game, but good title. They shot themselves in the foot by calling it Halo Infinite because... what happens after that? The context of the title is really important here, because if the game was called Halo: Infinite Evolved then the pressure wouldn't be that bad, but since it's just called Halo Infinite...
This is the same problem Call of Duty had with Infinite Warfare. That is by far the dumbest title Activision has used in the entire series due to the fact that it suggests the warfare goes on forever, so what's the freaking point? It's even more hairbrained when you take into account that there's gonna be another CoD coming out a year after this one, sooo.... In Halo Infinite's case, it suggests that we're basically stuck with these dumb Halos forever. Or the series itself is "Infinite", and will never die out despite the fact that it almost died 3 separate times since Bungie handed over the reigns. Maybe the title is a last ditch effort to convey to fans that 343 is committed to keeping the series alive, idk.
Additionally, the setup for this game is pretty suspicious given our Battle Royale ecosystem we got going on. If Call of Duty, and Fallout are cashing in on the hype, then I suspect Microsoft wants a piece of the pie as well. It would make sense for Halo Infinite since games like Rust, and Minecraft essentially go on forever (which Microsoft owns Minecraft now, mind you). We also gotta remember that Halo's been doing the Battle Royale crap since Halo: Reach, looong before it was an actual thing, and that we also have Warzone, a huge sandbox that easily could be altered for a Battle Royale mode. Combined with the generic title, classic artstyle nostalgia play, and apparent open world traits, I bet you Halo Infinite's gonna be a Halo: CE and Minecraft fusion Battle Royale experience shipping alongside standard modes. I hate the mere suggestion of it, but it's a rational evolution for the series.
(Oh yeah, forgot to mention... Animals, and nature were given allot of screentime in the Halo Infinite trailer... when have animals and their ilk ever been relevant to players in gameplay? Wanton destruction of Moa is fun, but it's a meme pastime. For these animals to be relevant, they'd have to be swole as hell, capable of killing any Spartan with ease. Or... conspiracy theory here... we're gonna be harvesting these animals and wildlife for resources... which reinforces the notion that Halo Infinite is a Rust-like experience.)