>>990062
Because Western Animation draws its roots heavily from problem-a-day formats, likely stemming from earlier Disney and Loony Toons shorts that revolved around slapstick and a failure to develop a deeper formula.
They would rather rely on maintaining a background 'continuity' of the occasional minor event being remembered rather than a building a consistent narrative. Heck, probably ties back into comics too, since during the silver age villainy had to be resolved by the end of the issue. And it ties into the three act structure.
Western cartoons are made for children, and unfortunately children are treated like children, by which I mean babies. 'How would they keep up with the story if the status quo changed?' is the usual logic, despite ample evidence to the contrary. This is probably why anime took off in the west so hugely. Stories, even the extremely drawn out ones like One-Piece and Naruto had story arcs, narratives, events, things that changed.
Romance is exceptionally hard to tackle in this regard, as young audiences, such as sub-10 year olds, are seen as uninterested in such stories (which may be true), while teenagers are… awkward as shit and develop at different rates, making it difficult to say when they are and aren't interested in romance. Just look at novels aimed at the 13-16 age bracket. Romance, be it a major or a side element, is rarely tackled at length or with realism.
In cartoons this is amplified by not having a god damn ending, which is the usual point that juniour fiction usually has two characters get together.
tl;dr
Probably inherent cultural problems with the format + demographic + what society permits, and an unwillingness to change.