That's exactly the reason why, especially of girls who get their start in Los Angeles/Porn Valley. These days all the studios want fresh faces, constantly. New faces are the only thing GUARANTEED to always make money for the porn companies.
New girls get lots of work for two or three months (many don't even last that long). As soon as you've shot once for each company, you're no longer new. In Southern California there are less than 20 studios shooting regularly. It does not take long to work for each of them once.
There are over 300 girls actively working in adult films and competing for a small amount of work here. If you're not brand new, you only get hired if you're really RELIABLE, attractive, a great performer, have a good attitude, and you're fun to work with. If you're not ALL of those things you won't work regularly and you'll "retire" very quickly and move on to other things.
That's why the mean average career of a new girl in porn is about 6 months. After that six month point is when it becomes much harder to keep earning income and getting shoots, and that's when the "wheat" (girls who are doing it because they love it, who aren't in it for quick cash) is separated from the "chaff" (girls who thought it would be easy money and they'd get rich doing porn). Still some of the chaff will stick around past that point, sometimes even for years. ...There are a small number of girls who keep working simply because they remain in demand, they're so ridiculously pretty or look like a barbie, but kind of hate their jobs. But other performers know who's in it for the money and who's not. Obviously it's a better job for us if we're working with scene partners who are doing porn because they like doing it.
