why i cannot get rid of 29 GB of "other" crap on my mac?

Because you cannot manage it - the OS now does:

https://support.apple.com/en-gb/guide/system-information/syspf9b375b9/11.0/mac/11.0

The Other category is a potpourri of files which can include:

• System temporary files

• macOS system folders

• Archives and disk images (.zip, .iso, etc. - often found in the Downloads folder)

• Personal user data

• Files from the user’s library (Application Support, iCloud files, screensavers, etc.)

• Cache files: browser, Mail

• Mail messages & attachments

• Fonts, plugins, extensions

• Safari reading list

• iTunes backups

• Crud resulting from jailbreaking your iDevice

• Game data

• Saved data files

• Call history

• Notes

• Media

• Voice memos

• Other files that are not recognized by a Spotlight search

• Media files that cannot be classified by Spotlight as a media file because they are located inside of a package

They can be located anywhere on your hard drive. The files that you have control over are located in the Documents, Downloads, Pictures, Music and Movies folders.  You can use either of these two free apps, GrandPerspective  or OmniDiscSweeper, to find the largest files on your drive so you can determine if they can be deleted or moved to an external HD for storage.  

Note: you can empty the Downloads folder after the apps and/or updates that were downloaded have been installed or applied.  Many users have found a couple of Gigabytes of files in their Downloads folder which are no longer needed. 

Locate backups of your iPhone, iPad, and iPod touch - Apple Support. This will tell you how to find any backups you may have.

Cheers,

Jack

check sync services for backups you may have made for the iPad, iPhone, etc. These accumulate tons of GB.

Since you do not know precisely what this "other" is comprised of, I suggest it is not useful to speculate that it is unnecessary. If you could remove it (and you can't, since it is part of the OS) you would find that your computer would not function. If you need more space, then the easy solution is to add an external drive for additional storage.

I had said that you cannot manage that category. The amount will fluctuate greatly and none of us have any control over it. if you wish to let Apple know how you feel about this "feature", you can do so here:

www.apple.com/feedback

.