It's a myth, they are not. They are uneducated and lazy¹, not stupid.
Save for minorities with genuine learning disorders, people can learn how to click/tap a bunch of buttons in a sequence. They aren't running a PKI (that's on platform/browser vendor), they are just picking an identity from the list after all. They learn how to click the right buttons all the time, as UI designers' managers decide it's time for a new bonus and swing things around with a "new graphic language".
Basic certificate management (from end-user perspective) is not harder than password management. Passkeys are conceptually the exact same thing² as TLS client certificates, just without a purposefully neglected UI and a DIY attitude³ (TLS is supported by nearly anything, barely anything knows about WebAuthn JS shenanigans).
1) Not in a bad way. "Lazy" as in "lazy evaluation", not "lazy ass". Maybe there's a better word for this.
2) Save for some technical details, both are essentially keypair management.
3) There is almost no UI, it's just an API then all the visual bits are left out as an exercise to individual app designers (partially on platform/browser developers, partially on website developers). Browser vendors just hated those areas because they weren't deemed cool and fancy ([un]like some JS framework of the day) and shoved them under the rug as deep as they could. And unlike TLS, there is no standard how to pass data around, everyone invents their own `POST /login/webauthn` semantics.