If database of the service A leaks, you will expose the password for all other services B, C ,D, E, F and G as well.
That's the assumption, that's why it is recommended to have different passwords. But technically, websites especially newly created ones have standard way of hashing passwords like Bcrypt or Argon2i which cannot be decrypted or what you ever call it. But let's just assume that's that for safer routes and practice security when creating passwords.
Yeah, hashing passwords is the best practice. But many websites do not follow the best security practices (and we often create accounts in poorly developed websites), and passwords do leak. Even some password managers find a way to leak passwords, unfortunately....