As you mentioned one reason is the interaction.
Like we all beeing here to get information or to share information (text or pics) and all of us has to decide if it brings us further or not.
So if a model or someone else see no need to promote or communicate with others, there is no need to be here.
It's not meant bad, it is rational.
And a lot of models don't want to have interactions with fans.
It's the same like in other business, companys don't want to interact with their costumers.
I bet a lot of them have several accounts too, so they can be around without drawing any attention.
I call them the "read only" members.
So be always polite to each other, you never know who sits on the other side of the web. 
Exactly, quite insightful IMHO. Seldom is a thread started by the person it is about, so their participation should not be expected. Anyone familiar with their past ventures into social media understands why they put themselves behind a nominal paywall; they get to limit exposure to those appreciative and ban those that are disrespectful.
Wolpertinger, I think you are making the same mistake I made for a number of years; assuming this as a business model. While I cannot profess to knowing how many have a open, free social media presence, I know Beshine removed all of hers a decade ago, no doubt the disrespect having played a huge roll; it is not something she takes lightly. Instead of assuming it is a business, think of it as a hobby, where the nominal fees cover only the costs, and then it makes sense. TBH, I'm not sure that Beshine covers the costs unless she gets a discount on those like five star suites she rents, probably for at least a few weeks at a time. I'm not saying that is the motive of every OF account, but with my past guesstimates of membership numbers to either Beshine's or Foxy's OF accounts, the yield is far less than most would assume. Others, with perhaps a thousand members of more, may be turning a good profit, but in all likelihood, those people also have a strong social media presence. I once pointed out the dramatic disparity between the number of followers Beshine and Foxy had on X compared to a few models with boobs far smaller, who also had OF accounts, causing me to opine that apparently, in the eyes and minds of the world, you can have too much of a good thing.
While Foxy like to attend events and mingle with her fans, Beshine seeks a private life. You would find the same on their OF accounts; Foxy interacts, Beshine doesn't. I don't see either trying to promote their OF accounts, but I do occasionally post things like this here for a couple reasons; to show support of our enormous boobie greed girls and to bump their threads in a way that at least attempts to offer something beyond a
and a picture. Some of the girls that are the epitome of what this site is supposed to be about go very ignored, but tastes differ, so no one is necessarily to blame.
The bottom line is as Malik says; if they were trying to promote themselves, they would likely be here and posting across many social media sites; they aren't. I think the mistake is assuming business is their intent, where it makes more sense to see it as a safe space to reach out to fans. Scanning through the comments on their threads shows the BEA is not a safe space.