Why do romance heroes tend to have a more substantial character arc if the protagonist’s character arc is supposed to be what the story is about? Obviously this isn’t the case for every romance book but you’ll find way more static protagonists than love interests in the mainstream books
I think it varies a little bit. The most common denominator is probably, frankly, that the author cared more about the (male) love interest and so gave him more interesting things to do and more emotional growth. And that genre romance specifically often frames the protagonist as like... simply a vector through which to carry out a romance with the love interest. It's super glaring rn with the way mainstream romances copy paste "ships" but usually the ship is like "ambiguous female protagonist/recognizable love interest from a well known IP" like all the ACOTAR rip offs are copying Rhysand, they're never porting over Feyre. There's an entire industry of Adam Driver/Kylo Ren love interests at this point, but even the super aesthetically blatant Reylo ones tend to barely have the most passing, surface level interest in Rey. But setting aside the "authors hate women" side of things, I think a love interest going on a huge, fraught emotional journey for the sake of the protagonist, is itself part of the fantasy. You know, it's "I can fix him" done completely unironically. The idealization of a man turning over a new leaf for the love of a good woman etc etc. Similarly passivity is like... not not framed as desirable in women either. The idea of the wide eyed ingenue who only has like three facial expressions?
It's still very Gender Roles, but that's genre romance for you!