Perhaps an example is in order:
Say that I want to add my sister. Doing due diligence, I search to find if her name is found. Now I know (because I contacted them all) that of my 13 living siblings (one more died young), none of them added a profile for my sister. But I also have 53 cousins, and a huge number of second cousins, plus 4th and 5th cousins that attend a large annual reunion of Oregon Trail descendants with me. Any of these could want to document my family, just to be able to keep track of us all, (How else could you?)
So I search. And I find six people with the same name, all of them private. Am I going to contact all six PMs with my information to see if there is a match? And wait up to six weeks? For each sibling? And cousin? And uncle?
And I myself, because I want to track MY cousins, enter their profiles (not all living; most are older than I). Repeat 53 times, plus parents to connect them, plus grandparents? I promise, I have done my best. And most of those whom I have entered would not have private profiles. (I have kept it that way partly for this exact reason.) But when profiles are private, and enough detail is given to distinguish, great. The rest of the time, well....
Now repeat 13,000,000 times for all the other genealogists, and you have a huge disguised problem. And soon it won't be a private problem (!).