Why can't motivations in a paper's introduction be copied?
At a fundamental level, every new paper is (or should be) presenting something new, whether it be new results, methods, experiments or even summaries or reviews. The paper would be redundant if it didn't present something new.

The 'introduction' or 'motivation' section follows the title and abstract as the next link in the chain to motivate people to read your paper in its entirety. Since it's your paper that you want to motivate people to read, it doesn't make sense to copy the words intended to motivate people to read some other paper.

There may be parts that overlap with prior work (and other answers on the site suggest methods to deal with that), but if you want your motivation section to relate to the novel elements of your paper, it needs to be recast for each new paper.

Your question (especially per your update) is why this boilerplate text can't be either copied or left out altogether.

When I'm presented with a paper, I am not always motivated to read it at first, so it doesn't help for the author to jump straight to the equations or experimental results, etc. What does help is to start with common ground. This is the job of the preamble. You can't leave it out altogether for the same reason libraries include the first part of each book's Dewey Decimal Number on every book - it's needed when the work is separated from the collection.

As for copying, it's a matter of skill in the craft to keep it concise, yet useful:


  No one prevents you from copying a few sentences from your previous papers. Except that perhaps page/word limits will encourage you to jump as quickly as possible to your actual contribution. – Alexey B