Why is ritual so important to Muslim prayer?

Islam is what is known as an "orthopractic" religion, which is one that stresses the overt signs of faith in preference to the doctrinal beliefs (as is done in those religions termed "orthodox"). Obviously, the clergy would prefer that their followers both believed and obeyed in the correct way (that's what the "ortho-" part means), but each faith ultimately shows a preference, and Islam prefers the ritual.

Another way to express the same idea is to say that Islam has an "agenda" (list of things it wants done) rather than a "credenza" (body of beliefs to be accepted).