Theft: A $2000 laptop is an easy target for anyone with sticky fingers, and so is a $1000 smartphone. A Yubikey has essentially zero resale value, so you will not lose them due to random theft.
Durability: If you drop your smartphone, there's a pretty good chance you'll shatter the screen and buy a new one. You can play tennis with a Yubikey and it'll be fine. You can run it through the washing machine and it'll be fine.
Longevity: Laptops and smartphones generally only have a 3-5 year lifespan due to battery degradation, and many people will want to swap it for one with more storage or whatever anyways. A Yubikey will essentially last forever, and if you stay clear of the insanity that is Passkeys its Webauthn element can support an infinite number of websites.
Portability: I have a smartphone, a work laptop, a home laptop, and a home desktop. My Yubikey has USB and NFC, so it can trivially be used with all of them. Individually enrolling each device would be a nightmare, and having the credentials sync is a bad idea from a security perspective.
Security: If your device gets compromised, it's pretty much game over: the attacker can now log in to all your accounts, any time they want. With a Yubikey I have to physically insert it and tap the button for each login - which is relatively rare because active sessions don't tend to expire. This means I would have to actively participate in a mass compromise of my accounts, making it way more likely to be noticed.
Passkeys is like embedded Yubikeys, or, Yubikeys are like external passkeys.
The point of passkeys that the key is kept inside a separate secure computer running secure blobs, so user codes can't touch it. That sounds sketchy but contactless payments using similar embedded secure computer has been fine so this should be too.
A couple of other people answered you already in a lot of detail, so I don’t have much to add there.
But I do recognize that really is a legitimate question and it feels like Yubi would benefit from running more outreach / promotion programs with schools and companies. I never felt like I could justify spending $50 just to try it out(especially when it doesn’t have support in a lot of sites), but then they partnered with Cloudflare to sell up to 5 per person at $10 each. It was a no-brainer to try it at that price and I haven’t looked back
Would that be what passkeys would be?