I think you're just mistaken, then. Health and fitness absolutely is the driver.
They tried to market it as a fashion purchase in v1, and then gave up on that. It can still be fun with different straps, but I don't know of anyone who considers it a fashion purchase.
Happy to see data proving otherwise. I'm also more than willing to believe a lot of people will use some of the asserted benefits as a reason to justify getting the fashion item. But even the enthusiasts I know that get some of the health tracking rings and such drop off after a hilariously short honeymoon with the devices.
The only exception I have seen to this, so far, are active users that are more likely to have a garmin.
That is to say: while they converted some existing watch wearers, they greatly expanded the market.
What I'm not certain of is "why". Fitness is an easy answer (that's something an apple watch does and my Seiko doesn't), but fashion is also a reasonable answer (apple is a premium brand)
I think the true answer might be "defaults are powerful"
Airpods are generally not the best sound quality you can buy, but they're well-integrated with the apple ecosystem. You can assume they'll just work. I would be surprised if they are a fashion piece, but I suspect their dominance looks similar to apple watch dominance
Do I think this is the only reason at all? Probably not. I will agree that there are certainly many people that do get it for the utility alone. Happy to see numbers going over any of this, of course. Mostly all I have to go on are anecdotes regarding Apple watches.
And yeah, if you do care that much (amateur or pro athlete training) there's no way the A-watch is the primary device for you. Garmin is the king of the hill in that market.