This is a benefit, because running a registry is a technical business, and running a registrar is a customer service business. In the old days, if you didn't like the service, you needed to find another TLD; post 1999, you can keep your domain and change your registrar when you want. The TLD can redelegate the registry business too, but it doesn't affect the end customers very much.
There's only 2800 registrars, and popular registries likely deal with each of them, but that's not a large number. When registries need to contact registrars, it's manageable.
But not all TLDs operate this way. ccTLDs can operate however they like: some have evolved to a registry/registrar operation, some offer direct registration only, some offer direct registration and through intermediaries.