Why can't you combine unlike terms?

Well, the phrase they often use is about "Apples and Oranges"... If you add them together, you tend to get senseless answers. For example: to add 3 km + 300 meters, it is clear that, since 3 km = 3000 meters, the answers is 3300 meters. If you just go ahead and add 3 + 300, for an answer of 303, well, that doesn't make any sense whatsoever.