They cannot return a value. They cannot have access modifier. They cannot be static. They cannot be invoked using a Class name or object. They cannot be declared abstract. Really is that your question and welcome to the ranch
They're not really like functions at all, other than having brackets after them - which is just an accident of syntax. How would you write a function (or method) to do what while does? That is, continually execute the block of code following it until a condition is true.
If you want to see a language where all the control structures are methods, though, have a look at Smalltalk (there are probably others). There, for example, a for loop is a method of the Integer class, which takes a block of code as an argument and executes it n times, where n is the receiver.