Execute Immediate Alter User Bind Variable

The following procedure:

create or replace
 PROCEDURE ChangePassword
 (
     p_Name      VARCHAR2,
     p_Password  VARCHAR2
 )
 AS
   EXECUTE IMMEDIATE 'ALTER USER :a IDENTIFIED BY :b' USING p_Name, p_Password;
 END;
 

compiles successfuly, but when executed:

exec ChangePassword('TestUser', 'newPassword');
 

results in error:

01935. 00000 -  "missing user or role name"
 *Cause:    A user or role name was expected.
 *Action:   Specify a user or role name.
 

Why?

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