For someone who’s been playing for years and looks back on the campaign’s from time to time, I admit the campaign is really simple and it is common for newcomers to get bored quickly; I’ve seen this mostly with friends who try the series out to play with me or because I urge them to try it out. I completely understand that the campaign seems bland or boring, I mostly see it as a foundation to set-up the story and prepare players for the extended lore that happens around the campaign, stuff that you read online or seen in the books, rather than the story within the campaign itself because I find that if you only do the campaign, you actually don’t know that much about what’s really going on. But there’s also other factors that I can appreciate which I enjoy such as dialogue, moments between characters, the environment and music placement + selection. A guy on here awhile ago said that he really enjoyed Halo CE’s campaign more compared to the others because it felt really alien, quiet, mysterious and dreamscape-y and I honestly agree. I also love ODSTs campaign… I’ve since figured out that I’m more drawed in when Chief isn’t involved that much; at first I didn’t know why, but then I realised it was because I didn’t find it him interesting in the games, I guess until Halo 4 where they gave Chief some semblance of an identity or character development.
Honestly, I find the cutscenes and parts where things actually happen more exciting than the controlled gameplay. Take for example the terminals in each game, which are essentially snippets outside the campaign, though the campaign still serves as the baseline… I suppose. Also, as others have said, it’s probably because a lot of people have nostalgia for the series. Some of my closest friends who are new to the series have tried other games with a campaign/main story weren’t all too impressed by Halo; they said it was “OK”. Lmao😂
You may also be not feeling that entertained by the campaign since Halo characters are generally less interesting than characters from other franchises or Sci-Fi’s, mostly because the characters are largely one-dimensional within the universe, which is a shame because the universe they’re set up in seems so mysterious and cool. But a lot of people like how the characters are like that - e.g Master Chief doesn’t get any real “character development” till Halo 4. Bear in mind that this is when an entirely new company comes along. A lot of people were divided on the campaign (lots of lovers and lots of haters) mostly because it, as UnheardCARNAGE said, breaks a lot of story telling rules established by its predecessors. Some people don’t really need or want intensive characterisation and emotion, they just wanna be a badass and blow things up, or explore an alien world and learn about it, while being badass.
Although, some characters do undergo development within the series, like Arbiter, Rtas 'Vadum, Guilty Spark and Cortana (to some extent, however mostly in Halo 4). But characters at the forefront of the story like NOBLE Team, Sergeant Johnson, the Keyes family and the Prophets don’t exhibit much character development that makes them stand out in terms of real change - even by the end of the story. Bear in mind that I love all of these characters, but even I can admit that they’re pretty one-dimensional, at least in the games. The novels and side stories make an effort to develop or add some depth to characters outside the games, like Johnson, some Forerunner characters and Master Chief; so if you’re bored how they are in-game, you can watch or read some stuff about them outside the games.
I still think you should play through the entire series’ campaign, and if you don’t like it, then it’s just proof that the story and gameplay of the campaign isn’t for everyone. And that’s completely okay.