You vastly overestimate the caution of the average user. 99% of users never read the rules and don't bother to read wikis before they start mass adding/removing tags. Some even somehow manage hundreds of edits a day even without the extra tools.
These things often go quite a while before someone catches it, too, and it only takes one person making a mess of a tag to cause a domino effect of users that don't question or double check anything perpetuating the mess. We don't really want more messes than we already have on our plate.
As one example of something I had to fix, someone very early on tagged Ghislaine Dedoldia as wearing a bikini, and other users that clearly just lazily copied Related Tags or took tags directly from other posts continued to mindlessly tag all of her posts with bikini until I noticed and cleaned it up. After that, the bikini tagging almost completely stopped, because no one actually thought about what they were tagging, they were just copying someone else's mistake.
Now imagine if every single user had the ability to use tag scripting and were making dozens of edits like that at a time. Promoted users are hard enough to keep track of as it is.
