Because most of the database systems such as MySql takes more time to process and deliver such data, and you also need to write lots of code for storing and retrieving images to and from the database, where as database systems are much efficient in performing searching and traversing operations on millions of rows.
Why not to use file system for traversing and searching ? and Why to use for storing ?
Because most of the file systems are really slow to handle operations such as searching and traversing millions of files usually they are not build for that, mostly windows based servers are slow. Where as file systems are much more efficient for storing and delivering files to network, file system being part of OS, uses system calls for delivering files on the network which is really fast operation as compared to database systems.