Sure you can technically just use the default systems to make a RPG(not even using eventing to custom make "systems", nevermind scripts), but that would be a pretty crappy game.
People don't realize the event command, do exactial what different function in a code does(the engine just has many of the base funtions of code has, just translated into a real language, so people don't need to learn some new custom language, inorder to make a game.
Seeing how RPG maker mainly(the exception, is some of the major things common in all RPGs ever created, wich is hard coded in the engine, but even such things could be built upon or even changed completely, you can even change how the engine interacts with the hard coaded data in the database altogether, which makes even the major hard coded things, completely customizeable) only has the the most rudimentally base operations, It's extremely flexible, as It's left up to the developer how to put it all together.
The proc in designing systems/things in general for a game, is the exact same as it would be, if you had to code everything from scratch, the functions of coding, was just translated into english, that's all.
People have no idea how flexible RPG Maker is, all it is, is a development tool, how you use it, is completely upto the developer.
Also you can import your own stuff(sprits, music, pics, windowskins, exc), plus for the few limitation the tools does have, there's a coding/scripting option you could use on a "as needed" case, to bypass the limitation.
People are under the impression, just because it has a default system(which is extremely basic), and offers an array of pre packaged graphics/music, that must be all people can, and do use.
People that put a actual effort into making a game, doesn't just hit a bunch of ok propts, and boom done, you have a game, All RPG Maker is, is a development tool, how its used, is completely upto the person developing the game.