At most wikis, external links are displayed with an inline "arrow" character at the end (represented here as ">"). The link label therefore looks something like "Example>".

When you use arrow keys to move the cursor through the link label, it looks like this:

E|xample>
Ex|ample>
Exa|mple>
Exam|ple>
Examp|le>
Exampl|e>
Example>|

Example|> (with the cursor between the last letter and the external-link icon) isn't possible, which makes it feel like you can't change the 'e' at the end.