Adding an entry is more complex that modifying one. Any entry can be modified using the sandbox, which is largely for test purposes. Unprotected pages can also be more permanently edited. If you think a new article is required, you can request that it be created.
There have been a number of controversies around Wikipedia, and its vulnerability to anonymous, malicious or erroneous changes.
The issue of changes being made to John Seigenthaler's biography is perhaps the most well known
Other issues with the site have included politicians using their staff to remove negative comments
One consequence of these controversies was a change in policy that prevented anonymous users from creating new pages