Why term set pop up doesn't have "Add new item"

One of the common confusions using term store in SharePoint is:
 #1. who can contribute (add new items) to the term set?
 #2. why the term set pop doesn't have "add new item"?
As in this example:

Here is the mystery solved:
for #1. Who can contribute to the term set?
In Term Store (_layouts/15/termstoremanager.aspx) you have Term set group to which you can assign access to contribute by adding a user to:
1. Group Managers (high privileged)
2. Contributors

More here : Manage permissions and roles for term sets

for #2.why the term set pop doesn't have "add new item"?
Make sure that:
1. a user has permissions to contribute (see above)
2. the term set submission policy it open

3. the managed metadata column in the list has "Allow Fill-In" - Yes