Why is plagiarising words such a big deal?
Per your example, you are not, in fact,  "forced to waste time" finding another way to express the idea that another author had. You can just copy the passage down verbatim, but to do so without putting it in quotes and citing it is plagiarism. If you write a publication that has no quotes in it, you are essentially claiming that you wrote the entire thing yourself. If you have copied text verbatim, that is false. You are taking credit for someone else's work, which is rather frowned upon in academia. 

Authors should be deliberate and unambiguous in their language, so finding the right words to express an idea isn't always a trivial task. A lot of work can go into a sentence that is succinct, precise, and understandable. Your choices are to either find different words to express that idea, or give someone else credit for having done the work already.