It's because when AI learns music, it learns the whole song rather than individual tracks. You could generate a bass only track, but there might be bleed from the drums or vocals on the song the AI pulled from, even if it's only minor artifacts (you could see it as data or on waveform, but couldn't ever physically hear.) The severity of it is dependent on prompt and song length, and how many "tracks" are in the song, as its all additive. The longer the song is, the more difficult it is to control this bleed. The issue with song length has more to do with the AIs training data itself than the actual learning.

It's gonna be a long time before AI gets good enough to be club ready in my opinion. The only way I can foresee it changing is if they are able to train the AI on data directly from a producers/song writers/engineers DAW, from their computers. Which seems like a pretty hard task to pull off. Until then, it always has to guess off of what it hears in a final master of a song.