This is true for lots of programs. Try installing TeXLive or a similar LaTeX distribution from a package manager. Usually it pulls in a *-docs package that takes up well more than half of the download.
To be fair, though, it is very easy to customise TeXLive so that it doesn't do this; whereas, as far as I know, Mathematica offers no such option. To be sure, you can delete it afterwards, but I'm not sure how gracefully Mathematica deals with that; whereas TeXLive, for example, gracefully deals with incremental upgrades from a very basic installation. (I know, because that's exactly how I run it on my tiny SSD. My current installation is well under 1 GB.)
I didn't believe you, so I went to check, and you're absolutely right. That's amazing. What in the documentation is using up all that space?
There are several thousands of documentation pages with very rich evaluatable examples, many of which return graphics.
Surely the graphics that they return should be returned, live, rather than passively occupying space even while unread? (I don't just mean 'should' in the sense of "that's the way it ought to be"; I thought, though I don't know for sure, that part of the appeal of the 'live' documentation is that that's the way it is.)
Some stuff requires internet connectivity, some stuff takes a certain time to compute. If you had to wait one minute for all the stuff to evaluate when you open a new documentation page that would probably bad user experience. The appeal of live documentation to me is that I can modify the examples in place until they do what I need.
About 70% of the space is taken by documentation. You can delete that and read the docs online.