Why are web services known to have loosely coupled architecture?

Basically loosely coupled means that anything is less dependent with other.In webservice if it is fully dependent with other service then a huge change of both side will be required.

For loosely coupled in web service understand with e.g : A consumer of a web service is not tied to that web service directly. The web service interface can change over time without compromising the client's ability to interact with the service. A tightly coupled system implies that the client and server logic are closely tied to one another, implying that if one interface changes, the other must be updated. Adopting a loosely coupled architecture tends to make software systems more manageable and allows simpler integration between different systems.

it is so important to achieve and implement if you want  Accompanied  new technology for following reason :

Dependency injection allows a client the flexibility of being configurable. Only the client's behavior is fixed. The client may act on anything that supports the intrinsic interface the client expects.

Dependency injection can be used to externalize a system's configuration details into configuration files allowing the system to be reconfigured without recompilation. Separate configurations can be written for different situations that require different implementations of components. This includes, but is not limited to, testing.

Because dependency injection doesn't require any change in code behavior it can be applied to legacy code as a refactoring. The result is clients that are more independent and that are easier to unit test in isolation using stubs or mock objects that simulate other objects not under test. This ease of testing is often the first benefit noticed when using dependency injection.

Dependency injection allows a client to remove all knowledge of a concrete implementation that it needs to use. This helps isolate the client from the impact of design changes and defects. It promotes reusability, testability and maintainability.

Reduction of boilerplate code in the application objects since all work to initialize or set up dependencies is handled by a provider component.

Dependency injection allows concurrent or independent development. Two developers can independently develop classes that use each other, while only needing to know the interface the classes will communicate through. Plugins are often developed by third party shops that never even talk to the developers who created the product that uses the plugins.

Dependency Injection decreases coupling between a class and its dependency.

Web services are loosely coupled as a web service contract (interface) may change without affecting the clients/consumers (endpoints).

For instance, say a new method is added to an already published web service. The consumer of the web service (clients) can continue to use the existing methods. So, the modification in the web service has not affected the clients and hence it is loosely coupled