Hi J-E,
WikiTree is unlike any other genealogy site. While you can upload a GEDCOM for processing, the information is not automatically imported in the tree.
As you noted, we use a process called GEDCOMpare. WikiTree works on the principle that there should only be one profile per person that ever lived. As you start to go further back in time, this gets tricky because more ancestors are shared by others.
The GEDCOMpare process helps to ensure that duplicate profiles are not created by suggesting potential matches for your data. Only after you review those suggestions (then match or reject the suggestion), can you then begin to add the profiles.
It is sort of a manual process to add them still at this point, but the data from your GEDCOM is used to help build the profile.
For more information, see: Help:GEDCOMpare.