You can sync your vault between devices, but what's the point if the clients on those devices don't support passkeys? As far as I know, no KeePass app on Android has mature passkey support. That's not even mentioning more niche cases, like what if I want to log into a website in a browser on my TV? The cool thing about passwords is that they work on any device.

Also, remember when one of the maintainers of the passkey standard warned that KeePassXC users would get blocked by relying parties [0]? Would you allow tech companies to determine what password manager you are allowed to use?

0: https://news.ycombinator.com/item?id=39698502

KeepassDX has pretty good passkey support I think. At least, it works for me.