They're plenty organized. Besides, they're cavalry. It's about speed; agility, and flexibility. The more chaotic they
are, the better.
Rohirrim is also a small group, with the far majority of them essentially being milita.
The Dothraki would annihilate them on that fact alone. Remember, the Dothraki are feared greatly, with their homeland location being something everyone else was greatful for. If Rohirrim were so great, they would have had greater reach. They didn't.