FTP is an excellent one to many protocol for distributing files. Far too many to enumerate here, but we'll try. FTP sites are often just not there. Dead links to FTP sites are very, very common. Even if the site is not dead, FTP can be very frustrating, especially to the beginner.
Some sites are just too busy to ever get on.
Some are ratio sites. They require you to upload something before you can download. Others are banner sites. They are out to make money. Usenet can be relatively anonymous.