I always chuck this up to young and inexperienced writers that are just writing because they think it's popular and it's going to make them rich...and they're not entirely incorrect, look at the 50 Shades of Stepping on a Puddle with Socks On franchise. In the latter case I chuck it up to terrible writing by someone who probably doesn't have the tools to deal with mental stress or has not experienced a completely healthy relationship or has a very skewed view on what romantic love is. I chuck it up to unresolved TRAUMA, put it that way LMAO.
Anyway, I don't think writing these type of relationships is entirely bad if they are meant to be that way. One example I can think of is Olivia Pope and Fitz Grant from Scandal. The entire fandom is WELL aware of the toxicity of those two and so is the show, the two of them just destroy each other at any chance they get but I think it works like this sadistic, sadomasochist, demented power-struggle tango. The two characters have power but the scales tip every now and then to favor one over the other. When it becomes a problem and icky is when the scales never waver and one character is ALWAYS the one with the shorter stick. One can argue that both are equally bad, yes, but one is good writing about a bad situation, and the other is a fetish of supposed romance and love, which is not good as it becomes this cartoon of something that is not real, and when it is it's not good or healthy. And if it is a fetish it SHOULD state so, you never know what impressionable minds are reading, like children or vanilla soccer moms from Suburbia, USA, know what I mean?
When I see indications of a negative portrayal of relationships that are meant to be romantic I just stop giving those comics my attention, it's just not worth it. At the same time I start to think about HOW to write a good relationship, be it romantic or platonic, to counter all the garbage that's out there, truly. I think the only way to combat this is to ignore and to create something that's good, kind of drowning out the evil noise haha. Not saying every relationship has to be rosy and peachy, but just have WELL WRITTEN relationships between your characters. I think 50 Shades of Soggy French Fries tried to redeem itself by the end by giving the dude (forget his name) some sort of tragic backstory which would have been okay IF Anastasia (how could I EVER forget that ridiculous name) had left the guy, but she DOESN'T and now she looks stupid and this boat has too many holes, we are taking too much water! MAYDAY, MAYDAY, we are sinking!
It all comes down to having a well written relationship, and yeah, I think you got it right, you can't write well when you are a teenager and haven't experienced life and different people outside of your high school and your town and family, or experienced any form of real hardship. There's just not that much stuff to get from that like @LordVincent said, most of them are not experienced enough in life to know how to write compelling characters with human emotions rather than these animal-like actions of possession and control.
Edit: I would like to first say... woops for making this long haha, and second @indagold there are a lot of abusive relationships in bara and yaoi manga, most of the times in yaoi which is mainly written and illustrated by women and it is aimed to women. I find abusive relationships in bara to be more power struggles, like who tops and who bottoms, and one finally giving in to being bottom, but real life doesn't work that way, it's silly... anyway, but yaoi is more like possessiveness and this feeling of powerlessness that is very reminiscent of real life mentally abusive relationships.