I have an svg file which has been edited for a long time over many iterations, containing many layers, but eventually the file has just one layer containing a filled rectangle and a text box. Somehow lots of garbage has accumulated during the editing, and the file size is 70 MB. After lots of attempts to reduce the file size, eventually I realized I just needed to click the menu File -> Clean up document, and the file size collapsed to a few kilobytes. I'm wondering why inkscape doesn't remove redundant information from the file by default?

I agree in most cases you should open the airlock and flush this stuff into the void.  But there are cases where there's a useful pattern or filter or something in there that you want to keep, even if you're not currently using it in your project.  That's why it's not an automatic function.

There are some attributes that can be erased when performing operations. In the Preferences, Input/output>SVG Output.

Use at your own risk.