Because Japanese is a spoken Japanese pizza topped with Chinese writing on half of it but a few bits snuck on the other side, too. Or some better metaphor.
Single kanji -> stand in for whatever the Japanese word is, read it like the Japanese word, probably. Two kanji -> oh shit, maybe, if you’re lucky, it’s the Chinese reading of both. But sometimes it’s not, sometimes it just gets slapped on the Japanese word. And if you’re really unlucky with a word, they mix. Which is first, I don’t know, you don’t know, the Japanese might know, or they might just add the pronunciation right on there. Four kanji -> I dunno, ask a linguist.
Also, thank you for introducing me to that video. Why indeed.