Ecosystem and other reasons are mentioned already in comments.

One more I would add to that would be, providing these services and making huge number of users depend on it and habituate them to use it. Later in life it is impossible for people to learn new similar tool as they are already use to this and work can be done. In case, there is enough data that the individual won't think to migrate outside then they can charge for service and increase it subsequently.

Later it becomes hard for other competition to stay in market because user cannot let go this service and switch to new.