Why is plagiarising words such a big deal? Plagiarising the ideas of others is obviously a big deal. You are stealing the hard work of others and misleading your peers by pretending it is your original work. 

But why is plagiarising words and paragraphs considered a big deal? I mean, isn't the point of academic effort to transmit ideas and knowledge? Who cares what specific words were used to transmit it? It's not like we are writing poems here: the words themselves don't matter, the underlying ideas do.

For example, say you are writing an article, and you need to add a short section on some background information. Your original source for this information is some book by a guy called Juntao, in which he writes


  It is well-known that the K-Group is hyperdifferentiable and pseudo-geometric. It can further be shown, given certain assumptions on the continuity of the underlying complex manifolds, that the associated Einstein-curves are super-composites and universally dense in the field of measurable spaces. Indeed, this was shown by Hilbert and Plato in their seminal work on uncountable and analytically algebraic left-Sylvester Deligne graphs.


Say you need to provide this information in your paper. What’s the problem if you just copy it verbatim? I mean, I could write it in my own words, easily. But why should I be forced to waste time on that? Why is it a problem if I just use the above formulation? 

As long as you refer to the original material, such as by ending the section with


  For more information on this material and proofs of the stated theorems, we refer to the works of Hilbert and Plato [1840]. A complete summary of the material can be found in Juntao [1998].


then everything ought to be fine, isn’t it? And yet, there are question like this, where somebody wanted to "seek justice" just because somebody else stole a few sentences of his.
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.