Why are the Caliphates not talked about alongside other great empires/generals?

Growing up with an American education, the history of the spread of Islam has always been somewhat vague. The little I knnow indicates that the accomplishments of Muhammed and his immediate successors should be put alongside Alexander and his Diadochi or Caesar and Augustus. The relatively small Arab tribes conquered the Sassanids, pressed the Byzantines, took over much of North Africa and the Middle East within a relatively short period of time. The Umayyads would then expand even further. The "empire" would last, depending on how you defined it, through the Ottoman Empire--over a thousand years. The legacy is still felt by over a billion people the world over. So, if you have the time, I have three questions:

• How did the Caliphates accomplish their expansion and the rise of Islam?
• Should the Caliphates be talked about alongside other notable empires?
• Why are the Caliphates not talked about alongside the other empires or famous generals?