Direct Combining Beans

Can anyone tell me why we don't use a combine to harvest our beans in California. We grow mostly Large limas, baby limas, and black eyes. I saw a you tube video where in North Dakota they cut the pinto beans and then picked them up with a large combine (maybe 30 foot header). No sickles on combine. They don't windrow them. We cut the beans, then use a bean rake (windrower) to put them into windrow, then harvest with CB Hay harvester. Seems like we are wasting money running the rake. I don't understand why it is necessary to windrow them.