A - the service doesn't encrypt your data (or has access to your private keys), and

B - that you have access to the second factor which is used to reset your password (e.g. a backup email address).

Secure password managers do not have access to your private key (i.e. the encryption key), as such if you lose the seed there is no recovery.

In the case with other services (i.e. they can reset your password) there is a chance you could lose access to your backup email or phone number used for resetting.

Edit: some services do allow reset by proving your "identity" (e.g. bank), in which case you have to go through customer service and provide requested information. (I put "identity" in quotes, because nefarious actors can sometimes prove this too).