There are numerous reasons for that. To begin with, most everyone wants to write for the most popular publications on Medium and making it too easy to apply would mean getting so many applications per day that it would be all but impossible to vet them all.
What’s more, not all of the publications are interested in publishing stories written by other writers. You can, for instance, create a publication as your own blog and use it to only publish your own posts. Keep in mind that publications have been created and are run by individuals or companies who use them with their specific goals and purposes in mind.