Because even Blizzard themselves admitted due to "ClassFantasy(TM)" tons of content they prepared is locked to a specific class, so if you want to see the full experience of Legion, all of the lore, you need several if not all classes.
If there was no class campaigns, class mounts and other stuff that is class-locked and you could collect and see everything from 1 main, it would be a different story.
Then there is also the "limited time" to obtain various artifact variants / tints for transmog, that limited time is until Legion is over, unless we have 100% confirmation otherwise, it's safer to try to collect as many as you can, in case you switch your main in the future and then feel screwed over like people do now on MOP sets if they only did it on a class they no longer play.
So far what we know is you can use artifact look as a transmog after Legion if you unlocked it in Legion, so it's time-limited, class and spec specific content.
For collectors, artifact skins and class mounts were THE reason to level alts, no one really raids or hardcore grinds on 5+ alts except maybe super tiny minority, but collectors are a big part of wow (and any MMO) landscape, why would Blizzard add mount, pet, toy, wardrobe tabs if not to cater to collectors? People actually like this stuff, and Blizzard gets their money. Wardrobe unlock system again promotes having at least 1 of each armor type classes, which didn't exist pre-Legion, transmog wasn't something measurable "how much % completion I have".
I'm personally not a hardcore collector, I don't even have "lord of the reins" mount achievement, but I can see benefits from Blizzard catering to collectors, first they're the segment of casual (non hardcore raider non hardcore pvper etc.) playerbase who don't immediately unsub after they consume the frontpage content, because they stay to pursue their collection goals. Second, they fuel the economy both in gold making (they often spend lots of time playing the AH or farming since they're non-raiders non-pvpers) and in gold spending on big ticket items like rare transmogs, mounts, boosts for specific achievement / mount. Third, they often are the people who go back to old content to farm transmogs, pets, mounts which means they're less likely to run out of content. Fourth, they're probably the prime target of Blizz store with mounts, pets, transmog helmets and such.
Many MMOs live off collectors, I played GW2 and SWTOR and their "cash shops" were always full of transmog gear, which means, this sells and keeps the game running. I would expect it's no different in other games of this type.
Damn, Blizzard themselves is overhauling HOTS and "overwatch-ifying it" with loot crates, tons of collectibles etc. I bet that means only 1 thing - Overwatch system paid off.
Back to the subject of alts in WOW, I wouldn't be surprised if majority of "alts" are not being "played" for AP or gear just for achievements and extra collection unlocks, they used to have a point for gathering as well but now with flying herbs are getting more manageable in price. Well, there's also a niche to have alts for order hall missions, if they didn't want people to level alts to do this, why did they carry over the mission table system from WOD to Legion?