It's also really efficient, setting up the entire relationship with just the word "step-bro". Plus, it's taboo enough for the characters to say "we shouldn't", but not SO taboo that the viewer thinks they actually shouldn't.

It's also probably relatable to a significant portion of the audience. I wager a fair number of people have experienced a relationship or wanted a relationship stemming from that situation.

Through no choice of yours or theirs, suddenly you are spending a lot of time with someone. From some perspective 'relatives', but pratically speaking you have a stranger who is your age move in and you are both 15 to 17 and things happen.

I had a not-blood relation suddenly happen when we were both sixteen, and we ended up dating for a bit. A bit more removed than 'siblings', but close enough to spend a lot of time together.

I think a subset of people like it, and the rest tolerate it. So, if you label every porn video as step-someone with step-other, you serve both audiences.

What I have a hard time wrapping my head around are scenarios where step-dad, step-mom, step-sis, and step-bro are all going at it in the same scene. Under what circumstances do these four people live together?

That's my understanding of why it's so common too.

Though I suppose there's some reason for concern, if you're really opposed to the concept of it. I view it sorta on the same spectrum as things like 'facials' and how wildly popular those have seemingly become -- a filming norm from a long time ago that mostly aimed to show/prove that the guy had cum, turns into something people do regularly because they'd seen it so much in porn. Same for things like anal, or many of the more mainstream fetishes/content -- it becomes largely normalized by its prevalence in porn.

Sorta like how that Dingus guy, Mayor in Ohio, got caught sniffing his step daughters panties. Dude prolly thought it'd be the start of a raunchy good time, where she catches him and then admits her lust for his old flabby limp dick dad bod.