When any profile has just been created, it is NOT automatically connected to the tree until the family is eventually linked to someone who already has the box on their profile.
Sometimes this takes a short time to make this link, or only a few extra profiles, and other times it can take years - such as for Stephen Hawking - or LOTS of new profiles to make that connection.
Also because Stephen Hawking was still alive in 2014, there was no rush to "connect" him for privacy reasons. Now that he has passed on, there is a rush to get him connected.
Remember all of these profiles are public and anyone around the world can see these so we have to be somewhat careful of private information - especially for those who are still living.
As an example of a notable whom I still have NOT been able to connect to the global tree, you might want to look at Richard Carpenter.
Richard's profile was created in 2016, but his sister Karen's profile was created in 2015. Neither of them have been connected to the global tree yet, despite all the ancestors who have been added. Because I still cannot find any relative who is already connected!!! This is called an "island." That is just one example.
Also on the sense of where the global tree is - this global tree is on the Wikitree servers only. Ancestry and Geni and World Connect all have their own global trees. NOT connected to Wikitree at all.
Wikitree claims to be unique because it ensures that everyone has sources (or tries to anyway) whereas a significant number of people on the other trees do not have sources. Those trees do not have any serious oversight and management whereas Wikitree does.
I hope this helps.