I suppose then that you would explain why Alduin won't engage Dovakiin in combat at other points in the game is just for show, so the show can go on?
Remember with Delphine and the burial ground? There are other times, too, if you happen to be at the right burial ground at the right time, you'll catch him resurrecting another dragon, but he flies off leaving the other dragon to fight you.
He treats you with contempt. He speaks to you in dragon language and then laments you don't even know what he's saying, that you shouldn't be calling yourself Dovahkiin, then tells the dragon he just raised to kill you. It doesn't take absurd leaps to work out his actions after that event. Either he still thinks you're contemptible and isn't going to bother coming down there to fight you himself, or he's smart enough to
definitely not come down there to fight you himself. The former is more likely, since he immediately makes to kill you when you break out a damn
Elder Scroll and learn Dragonrend by tearing your way through time with it, showing that, huh, probably can't just keep raising dragons and letting the problem solve itself, might have to personally go get this 'Dovahkiin'. The storytelling is poor beyond belief regarding Alduin and the main quest, it's a classic Heroes Journey that falls flat on its face instead of having any real flair and flourish to raise it up.
Why do people massively overthink something so simple? This isn't even meant to be difficult. The two dimensional, designated bad guy, a**hole nature of Alduin is
one of the narrative flaws of the game. He's just a big scaly monster with "Endgame Boss" stamped to his face, so people just don't give a damn when they pound that face in at Sovngarde. Quite a lot of people
don't even realise they just blitzed the main quest. Alduin has approximately the same motivations of a guy tying women to train tracks, he has the depth of a rapidly evaporating puddle of pee. Main Bad Guy turns up at the start of the story to kill a bunch of people and set the scene, because he's a dick and we need to see that, inadvertantly saves the very Hero of the story, OMG that's a new twist, said Hero then goes on a journey during which he makes friends, learns lessons about himself and how he's special and other such things, is treated like a whelp by the Main Bad Guy, who is a dick dontchaknow, then the Hero proves himself, Main Bad Guy turns up for Final Confrontation, gets anus handed to him on a silver platter studded with precious gemstones, runs away to his Tower Of Evil shaking his fist and probably saying something about getting you next time Gadget, is pursued by Mighty Hero (Upgrade from Hero! Good going!), and is killed in Proper Final Confrontation. The End.
At least Dagoth Ur was erudite. At least he had motivations. Damn, the guy was happy to sit down for a conversation over tea and crumpets before the screaming and stabbing started. By the end of Morrowind you were pretty damn sure Vivec and the Tribunal were just as bad as Dagoth Ur. And Oblivion realised Mehrunes Dagon would be about as interesting an antagonist as....well, Alduin, so they used Mankar Camoran to give character to the bad guys and used Mehrunes Dagon as a kind of fantasy equivalent to a nuke instead, his one dimensional motivation was properly utilised by making him a personified apocalypse and shunting proper Bad Guy duties onto a guy who could talk in a slightly broader fashion and with greater motivational depths than KNEEL BEFORE ZOD!
Why did Alduin burn down Helgen? He's evil. One dimensionally so. Why doesn't he just immediately kill an enemy like any good, neutral or pragmatic person would instead of treating them with contempt and mockery up until the point that, ooops, the enemy has become massively powerful? He's evil. One dimensionally so. Why is he the only dragon with a unique, armoured, more spiky and more black look? He's evil. One dimensionally so. We do we end up not giving a damn? He's evil. One dimensionally so. His name is 'Destroyer Devour Master' for heavens sake!
Come to think of it, if Alduin got any more unthinkingly one dimensional (by taking out his
one line of dialogue), it would cause a paradox where he'd inadvertantly become more complicated through the mysteriousness of his silence.