However, it is clear that this does happen. For example, it is not uncommon for sexual chat to turn into phone sex. Also, of course, some people actually meet. Usually these people are very disappointed. This is because the online environment fosters what psychologists call projection.
Projection involves seeing what one wants to see - essentially filling in the gaps with fantasy.
It seems to me that the secrecy of OSA adds to the addiction. Yes, this is a major factor. Specifically, the perception of online anonymity has been widely discussed among those doing work in this field. It is a lot easier to look at sexual material in the privacy of one's office or home than to go into a public place and make contact with others. Inhibition is significantly reduced because normal social restraints are not present.