This is actually kinda strange. What's the difference between dreaming and entering the fade the way dreamers do, or with lyrium, or blood magic? Like why waste lyrium or blood instead of just taking a nap?
Well, being able to actually get to sleep is hard for some of us. I've never been able to take naps mayself- unless it's heat exhaustion, and even then.
More seriously, there's probably a significant element of control. When we've seen or heard of people using lyrium to enter the fade, it's generally been with a specific goal or target in mind: to find this demon, or that spirit, and so on. Falling asleep might get you to the fade, but it could well be functionally useless for finding anything specific: where you end up is outside of your control, and you are limited in your ability to change your surroundings. Using a deliberate ritual to enter the fade helps you ensure you reach your objective.
We don't really have a 'why' this occurs, we just know that it does. Being a dreamer isn't about having an extra power source to fuel something any mage can do- it's a special exception in relation to fiddling with the fade.