Why are the Susie and Friends motifs separated? They're the same motif. Friends is a slightly cheerier and extrapolated variation but it's very clearly the same if you just listen to them side by side. Friends even has the same upward modulation on the second repetition, and it's only used in proximity to Susie as a character (with the exception of The Diner Song of Best Friends, which is a parody of Friends, where she was relevant.)
I don't think they're similar enough to count as the same motif. To really see the similarity, you have to take only the first half of Friends, and only take the treble clef part; and then compare that to the first half of Susie. That means you have to ignore the bass part in Friends, which gives Friends the rhythm that it has, a significant part of that motif in all used songs it appears in; and the second half of each motif, which are not very similar. Even if you compare just the first part of each (again, only upper notes on Friends), they're not the same, though it's similar enough that if Friends was just that, I'd probably call them the same motif too. But again, that's ignoring like half the notes in Friends, the second half of each motif, and the rhythm.
I do think it's probably worth noting the similarity between the two on the page. In fact, that's exactly why I notated the Friends motif the way I did, with the part that somewhat resembles Susie in the treble clef bur keeping the 5ths (the Bs in the score) in the bass clef, instead of the way I'd actually play it on a keyboard with the Bs in the right hand. However, I think they're different enough to be listed separately. There's also the "I guess I'm in love" case, which has Friends, but changes the melody significantly while keeping the rhythm. It no longer resembles Susie at all, and if all that were lumped in together, we'd end up with a Ship of Theseus situation where a listed song has no resemblance to the "original motif".
@Vexodus It's Motif time!
yeah man idk
i dont think so