No Halo game has taken this long to make either
A new engine takes time to make, as well as developing it multiple consoles + PC.
This is being developed for the Xbox One family, Xbox Series X, and PC. Yes, games can scale but making sure everything runs smoothly on every system, and has the feature set expected for that platform isn't always the easiest thing. I believe the audio on Halo Reach PC still isn't fixed.
On top of that, you might not care about multiplayer, but that doesn't mean 343i doesn't care. They are going to spend time making levels, working on Forge, other game types, etc.
I am allowed to have my expectation
I never said you weren't allowed to have expectations. I just said no Halo game has ever been 15 hours long.
Hell, very few FPS are that long.
Call of Duty? Nope. Battlefield? Nope. Bioshock? Nope. Metro Exodus? Nope. Crysis? Nope. Half-Life? Nope. Doom? Nope. Titanfall 2? Nope.
Probably the two I can think of are Far Cry 5 and Prey 2.
But more development time =/= longer game. And longer game =/= better game/experience.
But yea, you're allowed to have your expectation.