Why declare interface property as an abstract?

Can we not just declare the property not abstract? Why it has be overridden when implemented?

Every interface is implicitly abstract. This modifier is obsolete and should not be used in new programs.
The keyword abstract is redundant here

Why is overriden?

Use an abstract property when you have no default implementation and when derived classes must implement it.

Use a virtual property when you have an implementation in the base class but want to allow overriding.

Use the override keyword to override a member. Mark the member as sealed override if it should not be overridden again.

Don't mark the property as abstract or virtual if you don't want it to be overridden.

Use the new keyword to hide a non-abstract, non-virtual member