True, MySQL is a simple and fast database, but the complete lack of any kind of functionality means that you often have to do a lot of scripting to do what real databases can do by themselves. Things like foreign keys, functional indexes, stored procedures, triggers.
Basically, if you want to do just a little more than "SELECT * FROM table" in MySQL, you have to do it in a script, where real databases like PostgreSQL and Oracle let you do it in the database, making it a lot faster and much more reliable.