Packages have dependencies and potential conflicts with other packages. If you jump a whole bunch of versions really quickly, you risk incompatibilities and skipping required steps between versions.
So for example say you’ve not updated for a couple of months. If you’re updating package X from version 3.5 to version 4, and it requires package Y of a certain version to do that, you can be screwed, because you may have access only to package Y (too old version) or package Y (too new version). Or if you’re jumping more versions, the creators have only built it anticipating for 3.5>4>5, not 3.5>5, which can break in the intermediate step for the same reasons.
Like… have you ever seen youtube videos where someone updates a computer from windows 3.1 to windows 10 or w/e? they go through each version individually until they reach the end. Imagine trying to install windows 10 directly on a 3,1 system using windows 3.1 software.
And because Arch updates everything, including system critical stuff on a rolling release model with all the other packages, you can break not only some random program but your entire installation.