Character radicals only used for themselves?

Apologies if this has already been asked before, but I was looking through some radicals in future levels and I found ones like 麗 (lovely) and 飛 (fly) which aren’t used in any other characters but themselves. What’s the point of turning them into radicals instead of having them be standalone characters?

In WaniKani, kanji can only be constructed from radicals, but radicals can be introduced “from thin air”. So in order to introduce the kanji 麗 and 飛 WK needs radicals. These kanji are not easily composed of other radicals (well 麗 contains 鹿 - deer - but other than that it’s a bit hard to see how to compose those kanji), so it was probably easiest to invent their own radicals for them.