Is not that they can't, it is just that link are not designed to be ID.
The ID is the maximum cardinality of an entity (informally the smallest set of values of such entity necessary to uniquely identify it), the link is how to find the entity.
What happen if you want to deprecated your API?
There are other way to share the link of a resource,like use headers.
Then again, from a strictly practical point of view in 99.9% of the case it won't change anything, but:
1. Eventually there will be cases where it does matter, and then it will be an huge mess.
2. Why shut yourself a door only to don't read an header and obtain exactly the same information?
The ID is the maximum cardinality of an entity (informally the smallest set of values of such entity necessary to uniquely identify it), the link is how to find the entity.
What happen if you want to deprecated your API?
There are other way to share the link of a resource,like use headers.
Then again, from a strictly practical point of view in 99.9% of the case it won't change anything, but:
1. Eventually there will be cases where it does matter, and then it will be an huge mess.
2. Why shut yourself a door only to don't read an header and obtain exactly the same information?