It seems that your average TV prostitute is far more attractive than your real-life one. Could the likes of Julia Roberts, Natalie Portman et. al actually end up in the sex industry? A lot of women are on drugs too, and the effects of that are hardly seen in them.
It's show business. Almost everyone is better looking than real life.
It would also depend on which 'type', for want of a better word, of prostitute you're dealing with as well, since there's a sort of social strata; your average 'streetwalker' or 'crack whore', after several years of poverty, drug-addiction and working on the streets, will come off worse-for-wear than your average high-class escort or high-end brothel worker, who are usually more expensive and therefore can afford more in the way of expensive beauty regimes, clothes, etc. They're also screened more closely by the people who employ and hire them with regards to looks and potential debilitating drug-addictions, etc. Of course, most women who work in prostitution are streetwalkers, and most streetwalkers who appear on TV and in film are far more attractive and glamourous than the real thing.
At the beginning of the film, another prostitute describes Vivian as "new." Vivian doesn't do drugs and she seems fairly conscientious about her health in other respects (i.e. the flossing). In addition, she seems to be an intelligent young woman (Richard mentions her intelligence and she herself states that she did well in school before dropping out). In short, she hasn't been at this long and she's clever enough to see and avoid the pitfalls of some of the working girls around her. Maybe it wouldn't have carried her through the long run, but it's definitely preserved her looks so far.
Television, at least, seems to be working on making this a Dead Horse Trope - especially police procedurals. (Not to mention reality shows like Cops or The First 48.) Streetwalkers are appropriately not-attractive and "escorts" tend to be attractive in a porn starlet kind of way.
As a good for-instance, under the police procedural heading: Criminal Minds, any time you see a gaggle of streetwalkers, they're dressed provocatively, but tend toward being unappetizing
For more of the Dead Horse Trope, watch Futurama. The most attractive prostitutes are a 40-year-old overweight male, a robot (without synthflesh), and an amphibian.
It can happen. Even reasonably-priced brothel workers can be extremely attractive. And, really, isn't that the point?
Julia Roberts example is that it's set as a storybook/fairytale type story. At the end, you're supposed to forget all about her being a hooker and him being the type of guy who would hire a hooker.
How, exactly, are you meant to 'forget' that she was a hooker? The film deals with this right up until the last frame.
It seems like almost all important women in any media will be very beautiful (unless when they're unattractive, then you know they're evil), however men can vary in appearance. That hardly seems fair, as in real life you don't get a whole world of drop dead gorgeous women walking around just waiting for the right "hero" to win their hearts. If that were the case, anyone could make it into the modeling business. Also, why is it that pretty women are always separated into two categories? Either they've been chased after men all their lives, or no man has ever asked them out because they've been too intimidated. Is it that impossible to have a balance?
It probably has a lot to do with most writers and TV executives being male. Most of the guys you see on television rank considerably higher on the attractiveness scale than your everyday man, though just not quite to the extent of women. The only time guys on television seem to drop below "above average" is in comedies, where they still have a charm to them if not the supermodel good looks of a lead action movie star.
There is a trope for unattractive men who can score gorgeous women by their charm, intelligence etc. Kavorka Man. Gender Flipped female examples are extremely rare.