Basically, because they are three different entities: Findmypast and Ancestry UK are through private companies, while FamilySearch is through the Church of Jesus Christ of Latter-Day Saints. There may be SOME sources that all 3 have in common (if so, check to see if the data matches on all 3. Sometimes one site may index the same record differently because the transcriber reads the handwriting differently). Other databases are proprietary to that particular site. No one site will have all the answers.