Welcome to WikiTree! I hope you find it as interesting as I do to contribute to the growth of our tree.
The short answer is that probably no one has yet taken the time to add the children, or done the research needed to determine who they are. Or they might be a childless couple. For long-time genealogists, the marriage itself is important as part of the life story of two people.
Those who work on WikiTree for any length of time know that the tree is a work in progress -- we'll never be "finished" with it. An example: as part of a one-name study, I have created over 300 new profiles, and I'm still not done. My starting point was a particular immigrant to America who arrived pre-Revolutionary War. Profiles for him, his wife & children already existed, but profiles for the descendants of several of those children had not yet been created. His oldest son still has just a wife & no children -- I haven't worked on that branch yet, but I know from searching for other family members that he had a host of descendants.