Why do radicals sometimes appear differently in kanji? Why aren't those just... different radicals?

In addition to what Belthazar said, they can also help you guess the pronunciation of a word. Often more complex kanji will have one part that indicates a category of meaning and another part that’s solely or mostly there to indicate sound (often the part on the right, if there are two radicals side-by-side). It’s not foolproof—it doesn’t necessarily apply to every kanji, and some sounds have changed over time so the “guide” is no longer correct. If you’re interested in learning more of these associations, it’s not perfect, but you might consider checking out the following userscript: https://community.wanikani.com/t/userscript-keisei-%E5%BD%A2%E5%A3%B0-semantic-phonetic-composition/21479 . You could probably find some more resources if you look around.