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.
