If you don't want to sync your credentials with a server, why are you using a server based credential manager?

Keepassdx is actively working on this

Loos like no one knows of Enpass

If they're not portable how would I for example login to an account while on my Desktop, if I set up the passkey on my Phone?

They should be portable. I don't know if they should be stored in the cloud though. Ideally you can open a password database in whatever application you want if you have the database and login information. Not allowing this is inconvenient and doesn't seriously increase security.

Having them not be portable ensures people use them less frequently. If I couldn't sync mine between my phone and desktop I probably wouldn't use it. The way I do it isn't built into my manager (KeepassDX), but I use Syncthing to keep it up to date on both devices. It'd be better if there was an official option for people bad with technology.

The user should be able to do whatever they want.

Admittedly, for some password managers, the passkeys are stored locally and are not accessible in the cloud unencrypted without the decryption keys that exist on devices you authorize.

This may still not make a difference for you though. For me, I consider passkeys, even stored in the cloud, to be enough for the vast majority, so I appreciate these vendors working to make passkeys more easy for the end user.

Them being portable makes them actually useful though for me, unless there was a way to use them from a phone to login to a website on a desktop/other device.

Being able to login into a password manager and use a passkey is great, passkeys need to become mainstream to get everyone away from passwords, but they can't be locked locally onto one platform or you have issues. The regular joe won't be backing them up from their iPhones or whatever.

I don't see why a local option wouldn't exist though, perhaps they will come once passkeys have matured further.

I guess you're better off buying a physical security key, which offers some guarantee that the keys cannot be exflitrated from the device.